Durarara!!: Shinra, Shizuo, Izaya, and Kadota were all in the same high school, which happens to be the same school the teenagers are in at the present. Shinra was also in the same elementary school as Shizuo and the same middle school as Izaya.
Neon Genesis Evangelion: Ryoji Kaji, Misato Katsuragi, and Ritsuko Akagi all went to the same college. Also, Gendo and Yui were the advisee and favorite student (respectively) of Fuyutsuki. Gendo and Yui get married, and Fuyutsuki becomes Gendo's second-in-command
Bleach: Various members of the Gotei 13 attended the Soul Reaper Academy together, and it's those relationships that end up informing much of the intrigue in the Soul Society Arc.
Subverted in Saint Beast, as there is one training academy in heaven and everyone did go there together but realistically only a few of them met during that time because they were learning different disciplines. Their personalities, however, did not change.
Mahou Sensei Negima!: Many of the girls of Negi Springfield's Class 3-A end up becoming members of Ala Alba, assisting Negi in fighting monsters and other mages all in an effort to find his father. Even those who don't become official members help during crucial battles even if they aren't entirely aware that they are actually fighting people/beings from another world.
Soul Eater: all the DWMA staff we're introduced to were at the school together. An early chapter has them reuniting after being split up after graduation/re-assignment. With the possible exception of Stein, they don't seem to have changed terribly much.
Marvel Comics and Wildstorm Studios tried doing this during the short-lived Heroes Rebornreboot: they had Reed Richards, Victor von Doom, Tony Stark, Bruce Banner, and Hank Pym studying together at the same college, even going so far as to call themselves the "Knights of the Atomic Round Table" before breaking apart. Reed and Doom (and Ben Grimm) all went to State University in the normal Marvel Universe, as well. This State U is located in upstate New York, it is not to be confused with Empire State University, the university in lower Manhattan attended by Peter Parker and others.
In some versions of Superman, (most notably the Silver AgeSuperboy stories) Clark and Lex were at school together in Smallville. This is currently canon again, presumably to tie into Smallville (where they weren't).
There was a Silver Age story where Batman and Superman went to school together in Smallville. A Reset Button was pressed near the end when one of the characters was hypnotised to forget the other. The fact that it showed Bruce Wayne's parents to be alive and alright was the least of its many, many flaws. This story was later retconned in a late 70s anniversary issue of "World's Finest Comics" (a Roy Thomas written tale summarizing all the various Golden Age and Silver Age "first meetings" of Superman and Batman) to show it was Bruce's guardians, not parents.
Most of The DCU's plant-themed characters were Ret Conned into studying botany together in Gotham.
Chris Claremont in hix X-Men stories would occasionally do this with Professor X. At one point he established that he and the man who would become Magneto were briefly friends before splitting over their opinions regarding the future of mutantkind. In another flashback, set during the Korean War, he revealed that Charles Xavier served in the Army alongside Logan (before he became Wolverine) and Carmen Pryde, whose daughter Kitty would become a member of the X-Men as well.
Reimagined Enterprise: In a minor example, McLeod and Satterthwaite's ancestors met once during the Postatomic Horror, although their descendants weren't aware of it until they shared family stories in "Remembrance".
Because Beacon Academy is so central to the plot of RWBY, Fanon commonly portrays Ozpin, Glynda, and Cinder as having been students there. note This theory was jossed when it was revealed that Cinder was much younger than what the fandom thought. Nowadays, such fics are less common.
In the Adam Sandler comedy Big Daddy, several of the film's characters are said to have attended Syracuse University together. Even more improbably, nearly a decade later they're all still hanging out together in the same neighborhood in New York City, which, in case you haven't heard, is one of the most enormous cities in the world.
Terminator 3: Rise of the Machines of all things. While it wasn't specific said that all of John Connor's lieutenants went to school with him, it would appear that most of them did. From this we can conclude that the future ran out of professional soldiers and he had to fall back on people he knew for less than a year when he was ten (or thirteen, depending on which film you're watching).
In the third Austin Powers film it turns out Austin, Dr Evil and Number Two were all in college together. Also, Austin and Dr Evil are brothers.
Harry Potter: Justified in the series since there's, you know, only one Wizarding School in Britain. Still, it's a rather interesting coincidence that so many important characters (James Potter, Lily Evans, Sirius Black, Remus Lupin, Peter Pettigrew and Severus Snape) not only knew each other at that age, but were all in the same year.
Then again, many of the characters were only important because they knew each other and it makes sense they would befriend people the same age. Peter Pettigrew, for instance, would have drifted off into obscurity in the eyes of the Death Eaters, but because he was a friend of the Potters' he became important. In this instance it's less coincidental as it is causal.
In the musical, that's also where they met the younger versions of the Tin Man, Scarecrow, and Cowardly Lion. Glinda even lampshades this in Act 2.
Soon I Will Be Invincible: Played surprisingly straight (and given a small lampshade). Dr. Impossible, CoreFire, Blackwolf, and Damsel went to the same school, and mostly knew each other; however, it was a school for the intellectually gifted and they weren't all in the same year. (Also, it eventually turns out that Lily, though supposedly from the distant future, actually went to college with Dr. Impossible and CoreFire.)
Wing Commander: A lot of named characters in the novels tend to get this, though usually it's more that they knew each other from previous commands while serving in the military, not from schooling.
Stranger Than Fiction: The Life And Times Of Split Enz shows that the entire band, with the exception of latecomers Malcolm Greene, Nigel Griggs and Paul Hester, all knew each other from either the same high school or the same college, even if they didn't always go at the same time (e.g. Neil).
The Shadow branch of the Enderverse has all the major militaries of the world be headed by their respective Battle School graduates. Members of Ender's jeesh are the most highly-prized, with the ones who have no obvious Great Power loyalties (primarily Bean and Petra, being particularly skilled and also from Greece and Armenia respectively) being heavily recruited, and Middle and Great powers jealously hanging onto theirs (e.g. Hot Soup for China). This is of course justified—the Battle School was intended to pump out military geniuses, and now that the international military doesn't need them, Earth militaries want them.
Partly justified in Michael Flynn's Firestar and its sequels, since a number of plot threads follow members of a particular graduating class, but even throughout other parts of the series, Witherspoon alumni keep turning up in odd places.
Live Action TV
Buffy the Vampire Slayer: Out of all the opening credits characters in the Buffyverse, only four (Tara got in opening credits once, but if she didn't, I'd still count her) of them were not either working at Sunnydale High, somehow able to be in Sunnydale High 24/7, Working with the Scoobies while they were in Sunnydale High, had entered Sunnydale High multiple times or going to Sunnydale High. Even Angel's secretary in Season 5 is the former Cordette and Ex-Girlfriend of Spike, Harmony. That's what happens when you go to school on a Hellmouth.
Most of the Time Lords you will meet in Doctor Who know each other, and at least the Doctor and the Master and the Rani went to school together at the Time Lord Academy and were taught by Borusa. The Time Lord Academy is described as a stuffy British public school type of place with different houses (the Doctor and the Master were Prydonians) and classes on cosmic science and the like. It seems to award British-style university degrees (the Doctor barely scraped past on the second try, while the Master always got better grades and even his degree was higher). Just never, evercompare it to Hogwartsin front of an old series fan.
On Wings, the characters not only went to the same high school, they all had their reunion together, despite being different ages. This was Hand Waved by saying the school was too small for individual classes to have separate reunions.
Semi-averted on The Steve Harvey Show as Lovita did not meet the rest of the cast until she started working at the school. Steve and Ced were college roommates. Steve and Regina have known each other since childhood. However, later episodes imply that Ced and Steve and Regina attended high school together.
This is how the cast of The Class are gathered. The main character, who first met his current girlfriend back in third grade, decides to throw a party and invite all their former classmates from that year. The main cast consists of a handful of the ones who establish/re-establish relationships, and happen to all still live in the same city.
The four members of the original cast of It's Always Sunny in Philadelphia were all high school classmates, as were most of the recurring characters, including The Waitress, Rickety Cricket and various others.
Veronica's mother went to school with Love Interest Duncan's mother and father: explored in "My Mother, the Fiend" and a plot point in other episodes.
Veronica's teacher and Trina's mother. They all seemed to be in the same year.
This is how the 60% of the cast of How I Met Your Mother got together. Ted and Marshall were roommates at Wesleyan, and Marshall started dating Lily when they were all freshman. Marshall and Ted kept up the roommate thing when they all moved to New York.
Played with on Friends. Monica, Ross and Rachel grew up together. Chandler was Ross's roomate in College and met high school Monica and Rachel. Phoebe and Joey didn't meet the gang until they were adults. (Apart from when Phoebe mugged Ross).
'Smallville has a main character who is in high school, it's no surprise that a lot of the characters are going to school together. However, the writers thinking "hey, wouldn't it be cool for Character X to have a cameo" turned it into this for the DC universe as a whole (okay, some of them aren't actually going to that particular school, but they're around and they know each other).
This is the main plot of Graduados. Not only all the characters in their forties have gone to school toguether, it is also the reason of their personal relations and the origin of most story arcs. They also include several flashback scenes of that time period (played by the same actors, characterized as teenagers)
Star Trek: The Next Generation, Riker and Geordi were in the same year at Starfleet Academy, Troi and Tasha entered a year afterwards and Data was the year after that. Not quite everyone, but a substantial number of the characters (5, there were 7-8 on the main cast at all times) were at the Academy at the same time.
This was a Twist, Plotpoint and Reveal in Final Fantasy VIII. It turns out that everyone in the party (sans Rinoa), plus Seifer, all grew up in the same orphanage, which was run by Edea. The reason that none of them remember this is explained by their reliance on the G Fs...apparently, extended use of their abilities affects the user's memories.
In Tales of Symphonia, half of the final party either attended the school or taught at it.
In the Silent Hill series, Alessa, Walter, and Claudia all knew each other as children because they were all part of the same cult. This is one of only a few things that ties the various storyline together.
The three protagonists of "Supreme Commander 2'' all went to the same (presumably international) academy together. Surprisingly, none of them are forced to fight one another during the new war; instead they all band together against the real antagonist who is stirring up trouble.
In Elf Blood, given that the main institution of the story, The Council, has an academy that raises soldiers from elementary school age through high school and beyond, it's not that surprising that many of the main characters have gone to school with one another.
In Sinfest, Of all people, apparently Jesus and Bluedid. Currently not known whether this is canon or a one-off gag.
The Venture Bros.: Pretty much every single important adult character went to college together: Dr. Venture & Brock (roommates), Pete White & Baron Underbheit (roommates), The Monarch, and one-off villain Mike Soriyama. This was lampshaded by Hank when Doc (in-story) mentions a weird kid with huge eyebrows who's obsessed with monarch butterflies.
Hank: You are not gonna tell me that you went to college with The Monarch, too! Where did you guys go, Super Crazy No-Way School?!?
Phantom Limb and Professor Impossible taught at that same college, Dr. Girlfriend was a student there (though she didn't attend at the same time as the previous group), as was Billy (though he was undercover to spy on Phantom Limb). Even Monarch Henchman #21...
On Camp Lazlo, Everyone Went To Camp Together. Lumpus and Slinkman, anyway, and Jane went to the girls' camp on the other side of the lake.
Kim Possible: Every guest scientist of the episode that isn't old and gray can be assumed to have gone to college with Kim's dad. This includes arch-foe Dr. Drakken (aka Drew Lipsky, who didn't actually graduate).
The FlintstoneKids features Fred, Barney, their future wives, and even Fred's future boss, all attending elementary school together, but in the original series (and the 2000 live action movie) Fred and Barney did not meet Wilma and Betty until they were in their 20s, just after graduating from college.
The Flintstones: Fred and Barney dated Betty and Wilma late in high school (which became the focus of one episode involving an old love letter of Fred's). One episode also shows Fred never having graduated from high school, let alone gone to college (though another episode had him enroll in night classes at "Prinstone").
"We're suspicious of any high school that produces no fewer than 12 supervillains in one graduating class, but admittedly it does put an interesting spin on those Council of Doom meetings to think they're really just high school reunions."
A good portion of the adult cast of Arthur went to school together.
From 2007 to 2009, three alumni of James Madison High School in Brooklyn, New York were all serving in the United States Senate at the same time, from three different states. They were Chuck Schumer of New York, Norm Coleman of Minnesota, and Bernie Sanders of Vermont (Sanders and Schumer continue to serve together to this day). Other prominent former students of that particular high school include Supreme Court Justice Ruth Bader Ginsburg, stand-up comics Andrew Dice Clay and Chris Rock, and Nobel Prize-winning economists Gary Becker and Robert Solow.
Many New York rappers who came to prominence in the 1990s either grew up around each other or literally went to school together. Notably, Jay-Z, The Notorious BIG and Busta Rhymes often found themselves battling each other at lunch time.
Many of the big name Generals during the Civil War were all from West Point, regardless of their affiliations with the Union or the Confederacy.
Of course the fact that these generals had been officers in the U.S. Army before 1861 helps to explain this phenomenon. Some Northern contemporaries, especially the "political" officers, who before the war had not been part of this close-knit community, viewed this with suspicion, in some cases even attributing defeats of Northern generals against their former classmates from the South as due to a conflict of loyalties.
The West Point class of 1915 was nicknamed "The Class The Stars Fell On" because so many members, including Dwight Eisenhower and Omar Bradley, would become famous generals during World War II.
The famous British private Boarding School Eton College tends to produce examples of this trope in the upper reaches of government, with the current Prime Minister and Chancellor of the Exchequer being alumni.
This applies to a varying degree to all "public schools" in Britain - also Oxford and Cambridge University - with reference to government positions and a lot of positions in business and industry, hence the expression "Old Boy Network".
It has been noted that one private school in South Australia has produced nearly all of the state's Supreme Court judges.
That same school produced at least two Nobel Prize winners, a premier, and several federal ministers.
In France this also applies to the Úlite institutions, especially the Ecole Normale d'Administration and the Ecole Polytechnique.
French Revolutionaries Maximilien Robespierre, Camille Desmoulins and Stanislas Freron all went to school together.
The fact that this was an elite school in the national capital helped.
This theme seems to carry through the Revolution - many of the Revolution's major figures knew each other very well before they became prominent. For example, Desmoulins, Georges Danton and Fabre D'Eglantine all worked at the same law firm before the Revolution, and Jacques-Rene Hebert worked for Desmoulins' newspaper in 1789, though neither would be much more to the Revolution than enthusiastic observers until 2 or 3 years later.
Going international, Goethe, Talleyrand, Metternich, Benjamin Constant and Maximilian von Montgelas all attended the lectures of Christoph Wilhelm von Koch (Christophe-Guillaume Koch in French), professor of law of state and history at Strasbourg University.
This is actually how Monty Python started, as a live comedy group; Jones and Palin met at Oxford, while Chapman and Cleese were in the same year at Cambridge and joined the Footlights, with Idle joining them a year later. After picking up a strange-thinking American on a Footlights tour in NYC, the building blocks of the Pythons were there.
Three other contemporaries from Oxbridge (Tim Brooke-Taylor, Graeme Garden and Bill Oddie) who moved in the same loosely interchangable circles and who might well have become Pythons themselves took a related direction when their groups finally coalesced: they became The Goodies.
Robin Williams and Christopher Reeve went to Julliard together, being the only two students accepted to the exclusive Advanced Program that particular year.
Bryan Singer, Christopher McQuarrie and Ethan Hawke knew each other in high school.
Three of the four original members of Pink Floyd (Roger Waters, Nick Mason and Richard Wright) met in college, while studying architecture at the Regent Street Polytechnic in London. The fourth original member, Syd Barrett, went to high school with Roger Waters, and Waters' mother was one of his teachers in middle school. David Gilmour also joined the band after meeting Barrett in college, as the two both studied music at Cambridge Technical College.
Genesis was formed by five teenagers who were attending Charterhouse School near London. Two of them, Tony Banks and Mike Rutherford, remained members throughout the band's run.
The founding members of America (Gerry Beckley, Dewey Bunnell, Dan Peek) met while attending a London-area high school for US military dependents.
U2 was formed in 1976 by seven students at a secondary school in Dublin. One never came back for a second practice, one more left after a few weeks, and in 1978 one more member amicably left, leaving it with the lineup it has to this day.