Ah, finally! A high school that includes equestrianism in the curriculum.
A school — often a Boarding School — typically housing students of high school age or lower, but of a size and structure resembling a college or university. Although a story may only follow a few characters, backgrounds and wide angle shots clearly show that the student body is quite large. In anime and manga, this often goes hand in hand with being a One-Gender School.
May or may not be an Academy of Adventure of some sort.
Not necessarily related to the many University High Schools in Real Life, or high schools that are attached to and part of universities for that matter.
The library building takes up a whole island and is so large that a school club is dedicated to exploring it. It should then be noted that standard equipment for this club includes rock-climbing harnesses and cables. This makes sense (*ahem*) considering the library features shelves hundreds of feet high, secret crawlspaces (lined with books, no less), and waterfalls, which miraculously don't cause water damage to the books hidden behind them. (A Wizard Did It of course.)
The annual School Festival is comparable in scope only to an amusement park (by which we mean Disneyland/Universal Studios level) with elements of a mardi gras, and has such features as a Ferris wheel, parades complete with giant balloons, giraffes, and elephants, a fighting tournament, several hot air balloons, a few blimps, parades, fireworks, concerts, Humongous Mecha, and a full-scale replica of l'Arc de Triomphe which wasn't there the week before and isn't there the week after.
The previous designs for the school went even further, with one of them being a massive, Tower of Babylon-style school building.
The Academy contains an elementary, middle and high school, each split by gender, along with a university. If one assumes 12 grades along with Word of God's confirmation of 18 classes per grade with an average of 30 students each, then doubles this amount to account for the gender split, that makes for roughly 12,960 students...not counting the preschool, university, and staff.
It even go further, there is schools from kindergarten to university, split by gender or not, split by religion or not. In fact, this a city dedicated to education (and magic) with everything to live here. There is only an administrator instead of a mayor.
It's mentioned in one of the anime that Mahora has about 30 THOUSAND students. Talk about size.
Nagi's school in Hayate the Combat Butler is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the drug store, but that's just peanuts to Nagi's school. At one point, she says it's even larger than Vana'diel. The school even has a train service running through it which Nagi explicity states is because it so incredibly large.
Kamizono Academy (an Elevator School) in Hyakko. To emphasize the point, the main cast spent the entire first episode wandering around the campus, lost. Even Tatsuki, who has been a student there for ten whole years.
Freezing has several schools all over the world under the name Genetics, in which their purpose is to train Battle Couples to defend humanity from monsters from another dimension. Though it's been explicitly stated that all Genetics are military bases, it's still set up as a high school boarding school, complete with top-quality dorms, cafeterias, classrooms, and so on. (The only things missing are the counselors).
The titular Stella Academy in Stella Jogakuin Koutou-ka C3-Bu. Among other things, it has its own sprawling airsoft range where the girls practise their skills.
True Cross Academy from Blue Exorcist is colossal. The center of the academy is either one giant building, or a small mountain re-purposed as the location for most of the academy's buildings. And then it has a city spread around it. And the academy isn't even full. The main character and his brother have an old dorm to themselves, and there are thousand yard long hallways leading to classrooms meant to hold some 30 students easy that barely have 10, if 10, students. That's the special classes. The regular classes are full, but the place is still huge. It isn't clear if the school has more than just a high school in it as well. If it does, it might very well be up there with Mahora Academy above...
Ashford Academy in Code Geass, which educates both middle and high school students, like in the British School system, which makes sense, given the series' diverge from history.
Marginal Prince takes place at St. Alphonso Academy, combining Middle and High School years, which is huge and located in the middle of a laurel forest that is almost as big as the whole island it is located on (called St. Alphonso Island, of course). The main bulding's entrance looks more like a church or castle than anything and inside, the hall presents a collection of paintings and other fine decorations museums would be envious of. The students live in big houses with only a handful of people per house and have their personal butler (one per house). All in all, the school is full of history and prestige. It's kind of justified, as it's a place for the boys of rich and famous families to study at, students need to fulfill certain requirements to enter the school (mostly money or a good name) and the fees they have to pay are probably tremendous.
In Sailor Moon, T.A. Academy is the school attended by Rei Hino/Sailor Mars. It's a private, all-girls Catholic elementary, junior high, and high school that was apparently established by missionaries. Although we don't see much of it, what we do see is very impressive; for one, its dining hall looks like a five-star restaurant.
The Xavier's School For Gifted Youngsters in X-Men and various spin-offs has been this from time to time, depending on whether or not the writers have bothered to remember the "private school" cover story.
In The Crimson Rivers, the ficitonal University of Guernon, one of the oldest such institutions in continental Europe, lies in a remote valley in the French Alps, and has its own hospital (the only one in the area, with maternity ward and a morgue), a sports stadium, a sizeable library, a direct link to the mountains (including a helicopter, a cableway, at least one snowmobile, and a whole mountain rescue section), and the university's dean parctically holds governance of the whole countryside. In fact Guernon are so big, old and remote that they even have their own eugenics programme and inbreeding traditions.
Hogwarts in the Harry Potter series. It helps that it's apparently the only wizarding school in all of Great Britain.
A plot point in Lemonade Mouth, where the principal's focus is on funding elaborate new facilities.
Some portions of John Irving's novels are set in fictional versions of his prep school: Phillips Exeter Academy in A Widow For One Year; Steering School in The World According To Garp.
H.I.V.E (The Higher Institute Of Villainous Education) in the H.I.V.E. Series is built inside a volcano, and therefore contains a mountains worth of facilities, such as a hanger, swimming pool, library, greenhouses, tech labs, hologram arenas, docks etc.
And Amberwood Prep in the spin-off series Bloodlines.
Live Action TV
The Elite Way School in Rebelde, a Mexican telenovela. Rebelde, in turn, was based on Argentinian telenovela Rebelde Way. The Elite Way School didn't make much sense there, either. The real location of the Mexican version is actually a Golf club!!!
Despite the episode always taking place in the school during school hours, the students of Ned's Declassified School Survival Guide spend a disproportionately large amount of time in extracurricular clubs, the cafeteria, the outside lunch area, the janitor's closet, the hallways... anywhere but class, giving the impression of having as much free time as most college students. (Subverted to some extent by the occasional teacher asking to see hall passes, which are produced as though from Hammerspace, with an appropriate sound effect.)
Pacific Coast Academy in Zoey 101 takes this to a literal extreme — exterior scenes are shot on location at Pepperdyne University.
Chilton, the alma mater of Rory Gilmore in Gilmore Girls is a co-ed, day school example. It's based on the real Choate Rosemary Hall in Wallingford, CT, but Greystone Mansion, where the school is shot, is significantly larger than the grounds of Choate.
Xavier Academy in Mr D, and by extension Citadel High School in Halifax, NS where it's filmed.
Dalton Academy in Glee, the all boys school and home of the Warblers, certainly has a grand interior, although the exterior has not yet been seen.
The fictional West Beverly Hills High from the original Beverly Hills 90210. Filmed at Torrance High School.
Greendale Community College of Community fame is partially this and partially an inversion. It's ostensibly a small two-year community college. When it serves the plot for it to have features more typically associated with a high school, such as lockers, it does. And when it serves the plot for it to have features more typically associated with a four-year college, such as a competitive (in the sense that they participate in competition, not the sense that they actually stand a chance at winning) football team, an elaborate dining hall, and dorm halls, it has those.
GURPSIlluminati University is a large, sprawling campus, which includes nine separate colleges, a large library, a museum, two separate power plants (one of which is a gigantic sunflower), a stadium, multiple dorms, a Frat/Sorority Row, and a Botany building placed within a giant tree. Due to numerous reasons ranging from restructurization to shifts in reality, the exact layout and number of buildings on the campus is susceptible to change, to the annoyance of the faculty and student body.
Justice High School from Rival Schools. Billed as the most exclusive school in Japan, it's the seat of future leaders and diplomats. ..But mostly supervillains.
Also Gorin High, billed as THE school for future sportsmen and sportswomen. It has sections for junior high, highschool (attended by Roberto, Shouma, Natsu and Momo) AND university (attended by Shouma's older brother and Nagare).
Yamaku High School in Katawa Shoujo is absurdly large and well furnished, though this is handwaved in the game with the school having very good funding. The size can be justified, however, since it's a school for physically disabled students and some of them would need a good amount of room to move around.
Maritsu Evil Academy in Disgaea 3: Absence of Justice takes it Up to Eleven. People who have gone looking for the Home-Ec classrooms have never been heard from again, and the seniors are considered a myth, due to no one ever being able to pinpoint the location of their classrooms. It helps that it's strongly implied that the school's campus is the entire world.
Gunnerkrigg Court, to the point where wide angle shots of the student body (or anyone outside Annie's class) are noticeably absent, though other students do apparently exist... somewhere. Notable facilities include animal cages big enough for dragons and a full monorail system, and at least two "Holodecks".
Superhero School Whateley Academy in the Whateley Universe. Seven dorms, a physical education building with a large annex, three swimming pools, two full-sized indoor gymnasiums, a cafeteria that's inside a geodesic dome big enough to seat maybe six hundred, several large school buildings, an astronomy tower, a university-sized library that has really restricted sections, a church, a massive campus store, and miles and miles of underground areas, including: a power plant, three combat arenas and a holographic simulation center, dozens and dozens of public and private labs for the devisers and gadgeteers, you name it. It's so big it has its own building for the security teams.
RWBY's elite Beacon Academy looks about the same size as a city. This can be chalked up to Rule of Cool, like everything else in the series.
Fillmore! did this with a middle school, with X Middle School being a sprawling institution that functions very much like a small city. Also not a boarding school. One episode identifies it as "one of the largest middle schools in the United States".
Highlights include the corn maze, multiple composting piles for the garden club, an ice rink and Olympic sized pool, RC car track, convention hall, pier, boat dock, and more club rooms than you can shake a stick at (pottery club with giant kiln, cooking club bigger than most residential kitchens, and a Safety Patrol will full forensic abilities). Mostly Played for Laughs.
It's so big that the hallways are wide enough to allow(and the school large enough to justify) the principal to drive around(or rather, be driven around, by her own personal Yes-Man no less) in her own personal vehicle in doors, with at least a meter or two to either side for student traffic. It also bears mentioning that there are lockers along the wall. Meaning that all the main corridors are big enough to have roughly 3-4 feet of locker counting both walls, room for roughly 4-5 middle school students walk comfortably, and STILL accommodate her own personal enlarged golf cart.
Whether or not Kadic Academy of Code Lyoko is this is debatable. It has a pretty small student body, but it does have a whole lot of buildings and land...
Considering the extremely tight historical connections between Kadic, the factory, the power plant, and the Hermitage, the school's environs most definitely count.
Beverly High in Totally Spies!, mildly justified as it is set Beverly Hills.
Super posh Morningwood Academy from Family Guy is one of these
In Adelaide, Golden Grove High School, Gleeson Christian College, and Pedare Christian College all back on to one another at a central point. That central point happens to be Golden Grove performing Arts and Recreation Centre. In the foreseeable future, these three schools might, with a few boundary changes (the local shopping centre and a nearby apartment complex), become an Elaborate University High.
Rostrevorused to be one of these. Apparently it used to own part of national park.
Oundle School is rather like this, most likely along with many other private English schools. Its various buildings take up much of the town centre, and it has at least one 'outpost' in a nearby village. And it has its own radio station.
Nearby Uppingham and Oakham schools are similar in amount of students and number of boarding houses.
Eton, easily one of the most prestigious, expensive and exclusive of British private boarding schools, is fairly private about its extensive and ancient facilities.
Alvin High School in Alvin, Texas took over a community college campus when the college moved to new grounds.
Some highschools share their campus with actual universities, meaning they also share buildings and occasionally teachers.
The Norwich Free Academy in Norwich, Connecticut, consists of seven different buildings in which classes are typically held (Tirrel, Cranston, Shattuck, Bradlaw, Frank, Converse, and parts of Latham), most of which have three floors, as well as a very large library (Latham), a building reserved for administrative offices (Alice House), Alumni House, and a museum (The Slater Museum), which holds one of the finest collections of Classical statue-casts in the US. The school is state funded but not controlled by the local school district, and provides education for over 2500 students.
Davis Senior High in Davis, California, has barns, restaurants, and a fully-equipped auto shop on campus, among other things, and can proudly boast classes and experiences most people don't even get even in college.
Similarly, Plymouth-Canton Educational Park in Canton, Michigan, consisting of three high schools on one campus, has a restaurant, an auto-shop, a radio station, a forest containing an artesian well, and a stream running crossing the entire campus. It formerly had a barn as well, before it was moved off-campus.
Newton North High School in Newton, Massachusetts just completed a $200 million (yes, you read that right) renovation.
Even though the page picture's caption is a joke, Madeira actually does keep horses on its ludicrously huge campus and offer horseback riding as part of its curriculum.
Phillips Academy, in Andover, MA, is situated on roughly 500 acres (2 sq km). In addition to a host of academic, athletic, and dormitory facilities, the school has several lakes, two museums, and a large wooded area.
Many private prep schools are like this.
Victoria College, Alexandria (in Egypt) was intended to be a sort of Middle Eastern Eton, and as such has about as elaborate a campus as a high school can have in the middle of a major city.
Many high schools in New York City have open campuses. Often, there is a (relatively) modest school building that is located right next to stores, parks, cafes, etc. that the students all have access to during the day. Sometimes, gym and other classes may be held in different parts of the city.
Newer high schools in suburban parts of California tend to consist of many detached single-story buildings, spread out over a large area. It's quite common for a large high school to take up just as much land as a small college, to the point that it's very easy to confuse the two when viewed from a distance.
Schools in the North Texas area fit this trope very well, just google images of Hockaday school for Girls, or Plano East Senior High school, they are all giant open campuses with nice courtyards/ponds.