Dunce Cap

When a character does something wrong (especially in school) or to show they aren't the most intelligent chap around, they're shown wearing a large, white, coned shaped hat. "DUNCE" in large red letters, or a large "D", may be added for humiliation related purposes, or to exaggerate it. The character will likely be sitting on a stool in the corner, staring at the walls, as a punishment.

A variant is the "donkey ears" cap, that was favored in France.

Dunce (earlier, "dunce disciple"), by the way, was derived from the name of John Duns Scotus (from Duns in Scotland), the Franciscan philosopher, by 16th century opponents of Scholasticism.

While it is long out of practice, like its Japanese counterpart Standing in the Hall it is still often used in cartoon culture for a comedic effect.

Examples of Dunce Cap include:

Comic Books

Bart Allen often thinks in Rebus Bubbles; sometimes, when he (or anyone else in the immediate vicinity) says or does something stupid, or he simply does not understand what the hell is going on, a bubble will appear with the person of scorn wearing a dunce hat.
The French "donkey ears" variant is occasionally seen in Asterix, when Getafix is trying to teach the village children.

Film

Happens all the time in Our Gang shorts when they're in school.
Pete's Dragon has the titualar character Pete get a dunce cap put on his head, among other things, by his teacher when he gets in trouble for things that Elliot (the dragon) has actually done as well as his seemingly farfetched claims that "It was Elliot!"

Literature

Though minus the dunce cap, in Anne of Green Gables, Anne Shirley is forced to stand in front of the class with "Ann Shirley has a very bad temper. Ann Shirley must learn to control her temper" written on the board behind her as punishment for smashing her slate over Gilbert Blythe's head for calling her "Carrots".

Live Action TV

On the old Comedy Central game show Win Ben Stein's Money, whenever a contestant would begin an answer with "What is__?", Ben Stein would slap a dunce cap on them and shout "This is not Jeopardy!"
On Street Smarts (basically a Game Show version of Jay Leno's "Jaywalking") you could challenge you opponent to answer a question that was asked of the people on the street. If they didn't know it they'd have to wear a dunce cap for the rest of the round. But if they did know it then you had to wear the cap.
El Chavo del Ocho: El Chavo, Quico and La Chillindrina were on remedial classes and Don Ramon and Dona Florinda were allowed to be present at class as spectators. At the end of the episode, Professor Jirafales was glad only one person in there deserved donkey ears. Don Ramon was shown wearing them.

Newspaper Comics

Calvin occasionally gets a dunce cap in Calvin and Hobbes.

Video Games

A cursed item in Nethack that causes your intelligence to drop when you wear it. Before being identified, it appears as "a pointy hat"... indistinguishable from the useful Wizard's Hat.
In Escape from Monkey Island, one puzzle is to do as badly as possible in a school so you're stuck with a dunce cap. You then have to wear it so you don't leave a splash as you hit the water during a diving competition.
In Mechquest, your character gets one upon arriving late to his Mech Combat 101 class (by 3.0456 seconds) on his first day at GEARS University, after thinking "At least she's not going to make me wear a silly hat."
If you try to summon an "Idiot" on Scribblenauts, you'll get a kid wearing one of these. He loves eating bugs.

Web Original

Mentioned in an episode of Freeman's Mind: "I'm going to make a dunce cap, and you're going to wear it."
The Lonely Island/SNL Digital Short "Shy Ronnie" features a kid with one of these sitting in the back of the room.

Web Comics

This Order of the Stick strip.
Hark! A Vagrant has a strip where someone dies in a dunce cap.
Seen in this Scandinavia and The World page with "IDIOT" written on both of them.

Western Animation

In Yankee Doodle Bugs, Bugs Bunny helps his nephew Clyde study U.S. history for a test, but unfortunately Bugs fails history forever. After Clyde gets home:

Bugs: Well, Clyde! How did you make out on your history exam?
Clyde: [Puts Dunce Cap on his head] Does this answer your question?

Happens in the 1938 Merrie Melodies short, Katnip Kollege, to the bespectacled cat hero, Johnnie, who lacks rhythm and can't swing.
There was a character in a dunce cap in Disney's animated series Adventures In Music, which consisted of two shorts, "Melody" and "Toot, Whistle, Plunk, and Boom". These went on to be the Disney Sing Along Songs opening. It was on the beginning of every video, seen here.
Professor Farnsworth gets a holographic one in the Futurama episode "The Duh-Vinci Code".
In Ed, Edd 'n' Eddy when Kevin calls Edd a dork for trying to teach the cul-de-sac common sense, Edd shoves a cap on him.

Eddy: What a DUNCE!! *Kevin headbutts Eddy*
Nazz: Boy Kevin, that was stupid.

- In a later episode when Eddy is walking home from school there is a dunce cap on his head.
One Treehouse of Horror episode had Homer Simpson make a dunce cap out of a torn storybook page, only to have it burned off after accidentally leaning next to a candle on the shelf above him.
Back when he was a kid, Professor Utonium was a prankster who once put a whoppie cushion at another boy's chair. The teacher forced the other boy to wear a dunce cap with "Class Clown" written on it as punishment while young Utonium became a Karma Houdini.
In The Owl House episode "I Was a Teenage Abomination", Eda has to wear one after losing a bet with King.