A number of teen movies, such as Dazed and Confused.
In Dazed and Confused adults do occasionally impact the story. One couple ends their plans to go on a trip so they can shut down a party that their son was planning on hosting in their house while they were to be gone. Another mother threatenens some seniors with a shotgun when they try to haze her boy. One stoner hypothosizes that the adults are complict or at least apathetic to the strange situation of the local highschool.
The 1976 cult classic movie Bugsy Malone is a musical comedy about 1920s gangsters played entirely by children. It launched the careers of Scott Baio and Jodie Foster.
Specifically the book had lastday be the 21st birthday. The Movie pushed this up (and avoided Dawson Casting by having lastday become the 30th.
This trope becomes literal in Jimmy Neutron Boy Genius, where the entire adult population of Retroville is abducted by aliens.
Very deliberate in Fast Times at Ridgemont High, there are teachers, but no parents.
The MST 3 K-featured City Limits took place in a future where nearly all adults had been wiped out by a plague.
There are "adults" in the film Class of 1999 — but they are robots: violent, bloodthirsty robots.
The cult film Gasssssss has a plot where the title poison gas causes everyone over the ge of thirty to quickly age and die. A very seventies youth culture oriented take on the idea. Not sure how this compares to 1968's Wild in the Streets.
Marc Laidlaw's awesomely weird Cyber Punk short story 400 Boys features street gangs of psychic teenagers battling posthuman giants after the classic adults-only pandemic, topped off with 'World War Last'.
The young adult series Countdown began with all adults — and all children, sparing only teenagers — vaporizing into puddles of black goo. The result is a world run by teenagers as ancient prophecies come to life.
Just wait until you're on the other side of Logan's Run.
Shade's Children, the occasional clues to the pre-apocalyptic tech level indicate that it was Twenty Minutes into the Future (at minimum) even before the Change.
There literally are no adults in Shade's Children, with one highly technical exception. The kids are forced to wage their guerrilla war in a post-apocalyptic world alone.
Jack Dann's short story "The Marks of Painted Teeth" has teens competing for territory and provisions in a post-apocalyptic setting. They've apparently developed moderate telekinetic powers and a taste for Carl Jung.
Michael Grant's Gone series begins with everyone over the age of 15 disappearing, so children are left to run the community.
The Fire-Us Trilogy follows a band of kids a while after a virus (which is, of course Only Fatal to Adults) has rampaged the town. Unable to remember their names before hand, they have christened themselves with their jobs ('Mommy', 'Teacher', 'Hunter'. It appears the older children have renamed the younger children arbitrarily. 'Doll', 'Baby', 'Action Figure').
The Enemy by Charlie Higson takes place in a world in which all adults have been turned into zombies by a virus.
Live Action TV
The Odyssey (a Canadian kids drama) took this to a very literal level. A main character who after receiving a concussion, found himself in a world where there were no adults. This is the start of a journey he makes through this strange land where he encounters weird societies and places while he searches for his Dad (long story; don't ask).
With the exception of one episode which ended with a couple of parental arms reaching into view from the edge of the screen and dragging two of the trio away.
There was also an inversion in an episode where they reached a wall that separated their world from a world of adults, where there were no kids
This was more or less the whole premise of Party of Five at the outset.
The Canadian teen drama Edgemont (Kristin Kreuk, anyone?).
New Zealand show The Tribe has all the adults of the world dying of a mysterious virus.
iCarly another "older sibling has custody," something that has to be vanishingly rare in real life, but this show is camp bordering on fantasy, so it works.
Peanuts, which generally did not even show adults in the panels. The kids are often shown REACTING to adults, but they are always shown talking to someone who is off-panel. Anything the adults do or say has to be inferred by the actions of the kids.
Charles Schulz is on record saying that the moment he did introduce adults — in the background of a tennis court — was a big mistake.
It was actually a series of strips in either the late 1950s or very early 1960s involving Lucy competing in an amateur golf tournament while being coached by Charlie Brown. She has to withdraw and go home or miss her bedtime, even though she was winning. The adults make up the gallery, and are drawn more or less realistically, which doesn't remotely work in terms of scale with the regular characters. They appear in the Complete Peanuts anthologies, the first time they've been reprinted.
The cartoons substitute the famous gibberish horn sound for adult speech. The animators' original plan was for adults to have no presence whatsoever like in the comic. This idea was abandoned when it turned out that without some indicator of their existence, all the kids seemed like they were talking to themselves.
One animation exception: This Is America, Charlie Brown! The kids experience historical milestones by, among other things, interacting with the Wright Brothers. (no explanation was given for their displacement in time.)
Other animation exceptions include the movies Bon Voyage, Charlie Brown and its "this is a sequel?" What Have We Learned, Charlie Brown?, both of which feature talking adults, though not always seen.
Also the original reason was they "literally would not fit" in the close up style of the strip; children would just be at most a half panel instead of being focused on.
Role Playing Games
During a mission in a Stargate SG-1 based RPG, our team went to an Adventure Town planet with no adults. Lampshaded by the Cunning Linguist wondering how kids managed to function as a working society. Then 3/4 of the team became children. And it turned out that The kids of the planet were the adults, aged down through Applied Phlebotinum that allowed our team to return to adulthood.
The Sonic the Hedgehog games and cartoons technically have adults, and some of the heroes are over 18, but there are an awful lot of characters, notably Sonic, Tails, Amy and Knuckles, who have no legal parents or guardians, and seem to live entirely on their own. In the post Sonic Adventure world, the games don't even have the excuse of Robotnik ruling the world and having roboticised their parents.
This was subverted in the comics, where gradually more and more parents turn up whenever the writers feel like having a reunion.
Originally in Rule of Rose there are whole two adults and one sixteen-year old in the Rose Garden Orphanage supervising over twenty children, and by the end they all have mysteriously dissappeared without anyone from the outside noticing, leaving the orphans to their own devices.
In the TV show Max & Ruby, the parents of Max and Ruby are never shown, though their grandma sometimes makes an appearance.
And while she makes appearances, she does not live with them. Ruby does all the mother work.
The stage show "Max & Ruby: Bunny Party" lampshades this with a song that is actually called "Where Are the Parents?" According to the song "They're busy making plans, scrubbing pots and pans / Writing letters, folding sweaters..." and "They're not too far way / They're on the sundeck just to relax / Not too far from Ruby and Max."
No adults appeared in Ed, Edd n Eddy. Later episodes are a little less strict with the rule, showing certain body parts and representing their speech with sound effects, but the only adult to make a full on-screen appearance was Eddy's brother.
One episode of South Park had the children all accuse their parents of molesteration, getting all of them arrested at once. Naturally the town goes to hell in a few hours.
Strawberry Shortcake, especially in the 2003 version, where the characters are designed and act more like real kids than the other versions. In this version, Strawberry Shortcake and Apple Dumpling are sisters, yet no parents are seen or mentioned.
Scooby-Doo. Those teens travel all around the world with their talking dog, capturing nut jobs in Halloween costumes. Don't they have to go to school at some point?Do they have a curfew? Do their parents care about enforcing this stuff? Do they even know or care where they are? Apparently not.
The very latest series, Scooby-Doo! Mystery Incorporated, in fact does bring their parents in; they very much do not approve (also, Frank Welker voices Daphne's dad. And he still does Freddy. Don't think about that too much).
Charlie and Lola. Though they TALK about "Mum" and "Dad," there's never any sign of them, not even Charlie or Lola reacting to them. This is sometimes taken to ridiculous and often frustrating extremes. For example, in the episode "Charlie is Broken". After Charlie breaks his arm, Marv informs him that his father is coming to help him, only for the episode to cut jarringly to another scene. Also, in the episode "I Am Really, Really, Really Concentrating" at the end of the episode, Lola recieves a unique rosette from a teacher for managing to participate in an egg-and-spoon race without her egg falling off her spoon once. She is shown to have recieved this rosette seconds after the race finishes, no teacher in sight to have given it to her. Charlie, age 8, could have been raising his little sister and has instructed her to talk about their parents (and grandparents), so that the authorities won't separate the children. However, Simon Pegg has been threatening. Although, someone must be driving on the rare car rides.
Justice League episode "Kid Stuff" had the heros become 8 years olds in order to exist in a world were adults were magically removed.
Young Justice had a similar episode, "Misplaced", where a group of evil sorcerers creates two separate worlds, one with adults and no children and one with children and no adults, based on the "World Without Grown-Ups" storyline.
In The Simpsons when Homer and Grampa distribute their "re-vitalizer" to Springfield, adults excuse themselves to their bedrooms, leaving the town children wondering if the grown-ups have become reverse-vampires.
In The Powerpuff Girls, a squadron of broccoli-shaped aliens attempts to conquer Townsville by contaminating the broccoli harvest, hypnotizing anyone who consumed the vegetable. This leads to the trope, since all the city's children consider it too disgusting to eat.
Winnie the Pooh should be mentioned here. In none of the original Disney animated shorts are parents, with the exception of Kanga, present, however, being a stuffed animal, I don't think she qualifies. However, this is subverted in The New Adventures, as Christopher Robin's mom, as well as other adults, appear in the animated series. Hell, she, and a theater usher appear in just the very first episode, "Pooh Oughta Be In Pictures".
If Kanga applies, then so does Lumpy's mother, deemed "Mama Heffalump", in Pooh's Heffalump Movie, and in My Friends Tigger & Pooh.
The original whole point of Winnie the Pooh was that it was the world Christopher Robin had created to play with his stuffed animals in. He's often away, of course, having other claims on his time, and at the end of the last book he's growing up. It's sort of a precursor to Toy Story, only without a real-world context.
Taken to extremes in Little Einsteins. Not only do we never see the main characters' parents, there doesn't appear to be any other humans, period.
In the short lived Flintstones spinoff Cave Kids, a Muppet Babies or Rugrats -esque show focusing on Pebbles and Bamm-Bamm's "adventures", their parents were no where to be seen.
Though there are (if mostly evil or incompetent) adults in the Codename: Kids Next Door universe, the rumored goal of the apparent KND Splinter Cell was to wipe them all (good and evil alike) out, however, it turns out to be just cover-up for the KND Galactic branch, much to the ire of some fans.
Although it's averted in the main series, My Little Pony Equestria Girls plays this straight with the human world — aside from Principal Celestia and Vice-Principal Luna, the only other adults are cameos with no effect on the plot (Granny Smith, Cheerilee, and Mr. & Mrs. Cake). The absolute lack of parents is puzzling; the total absence of law enforcement after a flying fire demon demolishes the front of the high school totally boggles the mind.