Oh God, Stephen King, what sadistic schoolkids inspired you to make such cruel children in your novels?
Possibly the worst example is in "It", where the main bully is sadistic, and has sexually harassed and threatened his classmates. He even goes as far as to molest Beverly and attempt to carve his name into Ben's stomach.
Stephen King's Carrie was picked on since childhood, literally outcast by her entire school, but it wasn't until she reached her teenage years that everything came to a head and she snapped and went on a rampage. No one except her crazy religious fanatic mother (who was herself abusive to her) knew about her telekinetic powers until it was too late.
"Please Stop Laughing at Me" by Jodee Blanco is a slightly fictionalized memoir of her childhood, which was full of utter sadists who threw her in the toilet, pelted her with rocks, and made her life a living hell. And that was just before High School.
A Series of Unfortunate Events: Carmelita Spats and her groups of friends in the book "Austere Academy" played this trope to the hilt around the Quagmires and the Baudelaires especially. Unfortunately for the Baudelaires Carmelita returns later on in the series to torment them further.
James Potter was a Jerk Jock bully, growing out of it only when he was out of his teens.
Snape's childhood is the epitome of this trope, since only Harry's mother seemed to treat him like a human.
Dudley Dursley, at least at the beginning of the series.
Voldemort was this when he was a kid. He tormented the other orphans by forcing a bunny to hang itself and scaring two orphans by taking them to the cave that he later leaves a Horcrux in.
The Slytherins that Harry learns with can be considered this. They enjoy bullying anyone whom they think aren't worthy, including all the Weasleys, Harry, every Muggle-born, Neville, and all the Gryffindors. No wonder Harry finds them to be evil.
The most unambiguous Narnia example is probably the school bullies in The Silver Chair, who have been tormenting Jill in unspecified ways at the beginning of the book, and whose approach is what drives Eustace and Jill to blunder in a blind panic of terror into Narnia. There's a whole author aside about how horrible the kids are at their school, which is called Experiment House, and how the adults do nothing to control them. It's pretty satisfying when in the end Jill and Eustace beat the crap out of all the bullies, with Aslan's blessing.
In Mercedes Lackey's Heralds of Valdemar novel, Brightly Burning, a troop of school bullies torment a boy who has the ability to start fires with his mind. Needless to say, it ends badly for them (and leaves the Firestarter wracked with guilt for some time afterward.) Not an example of Bullying a Dragon, since no one knew he had the ability until it manifested when they pushed him over the edge.
Partially justified in Arrows of the Queen where Talia Sensdaughter was involved. Yes a lot of the highborn kids outside of the Heraldic Trainees were displeased by some puny farmgirl from the borderlands receiving some of the best education in The Kingdom at the Collegium, and some of that could have been due to her being groomed to be the second most powerful person in Valdemar; but there was also an actual conspiracy reaching the highest levels of court that wanted the incoming Monarch's Own Herald driven off, driven mad, or just plain dead.
The classmates of the baby-sitting charges (especially Charlotte's classmates), though this is existent in the BSC's classmates as well, especially in Mallory and Jessi's sixth grade class.
Some of the charges have this too — though mostly they're of the prank-playing kind. One of Claudia's charges once played a prank where she didn't tell Claudia that the chain of a swing was broken, thinking it'd just break under Claudia's weight when she sat on it. Instead, it held, the kid forgot to warn her, and the chain finally broke mid-swing, leading to Claudia breaking her leg so severely, she had to stay in the hospital with the leg in traction. The rest of the book switched between Claudia recovering and the club joining forces with some of their other charges to get the kid to stop playing pranks.
A small part of The Subtle Knife discusses this: The protagonist's actions inadvertently cause the older brother of two kids from Cittágazze to be caught by the Specters. A few moments later, a big group of kids, many of them armed try to kill them. After they are rescued, Lyra is astonished at how kids are capable of doing such things. Will replies he already knew, due to having to deal with kid's reactions to his mentally ill mother.
Lyra herself, though, had plenty of expose to kids being cruel before the start of the book, not as a victim, but as a leader. The beginning of The Golden Compass details some of the things children do in Oxford. It gave a simplified version of the systems of often very short alliances, as well as rivalries, and explained that kids from different colleges would attack each other, or gang up on kids from the town or the bricklayers' children. They have fist fights and throw stuff at each other, up to and including bricks, shove the bricklayers' kids viciously in the mud (that description was rather detailed), and tried to sink the a Gyptian boat. Lyra and the rest of the kids, of course, viewed it as a game, and the violence was mutual. It still sounded very vicious and far from innocent.
Played with in Ender’s Game. Ender is picked on at his first school by the other kids for being so smart, and so small, and later for similar reasons at the Battle School, in a straight use of the trope. He shrugs it off. However, when his personal safety is threatened, he turns the tables on the bullies and Ends the threat. Permanently, by killing the bullies.
All Summer In A Day, the short story by Ray Bradbury involves a kid being locked in a janitorial closet on an extremely rare sunny day on another planet simply because she originally came from earth and claimed to have seen things like the sun and flowers before.
Eloise McGraw's The Moorchild. The other children pick on Saaski because she's different, and their teasing often turns violent and she ends up injured. When a prank could have turned deadly (an older, stronger boy tries to push her in a deep pond), nobody helps her. The children's parents deliberately look the other way, and when they stop, things get even worse for Saaski.
Blubber by Judy Blume. Even the main character partakes in the bullying of an overweight loner. When she gets bullied herself, it's no longer a laughing manner. Parents have been known to complain that no one gets punished at the end.
This is the driving theme of Margaret Atwood's novel Cat's Eye: the protagonist suffered severe and permanent psychological damage, including tendencies to self-harm, from being cruelly bullied as a child, and her later career as a painter reflects the pain from this period of her life. She's also depicted as terrified that her daughters will suffer similar abuse, or torment others, at that age.
Atwood depicts a similar history a little more lightheartedly in Lady Oracle, where a casual social encounter as an adult with a woman who used to bully her when they were both children (and now doesn't remember it at all) drives the protagonist, Joan, to retreat to a restroom and cry.
Little girls are small and cute only to adults. To one another they are not cute. They are life-sized.
White Fang has Puppies Are Vicious. The "puppy pack" gangs up on the protagonist, making him grow up to be a cunning and brutal fighter.
Pick a V. C. Andrews novel. Any V. C. Andrews novel. (Though yes, most of the books actually have Teens Are Monsters, a fair number of them have much younger bullies as well...special mention goes to young, frail Carrie's classmates, whose merciless bullying (even locking her out on the roof of the school once) played a part in her being Driven to Suicide.)
In Under the Dome, Julia tells a story from her childhood about how a gang of girls beat her up and stole her pants. Near the end of the novel, it's revealed that the Dome is the product of Eldritch Abomination "children" torturing the town in the same manner and attitude that kids torture insects.
Protagonists Emmeline and Adeline in The Thirteenth Tale have zero empathy for other people and casually destroy things and endanger infants.
In Inheritance, the final book in the Inheritance Cycle, Galbatorix has a couple of children kidnapped, and states to the heroes that he's never believed that children are innocent, just that they usually lack the means to act on their cruel urges. He then tells them that if they believe that children are innocent and consider themselves virtuous, that he'll kill the children if they choose to act against him.
Subverted in the children's book Angel Child, Dragon Child. Ut's family moves from Vietnam to America, and on her first day of school, she and her sisters are taunted by the other children. Finally, Ut snaps and gets into a fight with a boy named Raymond, and the principal makes them sit down and talk. Eventually, Raymond understands Ut's problem — that her mother wasn't able to come to America with them — and he and the other children organize a fair to raise the money needed to bring Ut's mother to her family's new home.
Taken to an art form in Jerry Spinelli's Wringer. Beans is nine when the book starts and he's a borderline sociopath. Mutto's just as nasty, and even the protagonist Palmer can be cruel if coerced into it.
Brokenstar of Warrior Cats had been revealed to be picked on by Runningnose and his siblings (who enjoyed taunting him about his mysterious mother) and they called him "badger-stinky". They keep this up until Yellowfang finally yells at these kits to stop picking on him. They try itagain (except Runningnose, who learned his lesson), but they end up being pushed back by Brokenkit.
Socks and Ruby did this to Scourge (once named Tiny), believing he was too weak to fend for himself. Karma catches up with them when their Twoleg owners abandon them and Scourge refuses to take them into BloodClan.
In Jury Macnitier, after Jury is discovered to have no magical powers, the other Angel kids tease her mercilessly, so Jury is lonely and friendless for most of her childhood. She's also only THREE years old. She does eventually find friends, and later on discovers that she DOES have magic, and is in fact a really powerful level Angel.
Almost ALL books written by this author are depressing. Remember Courtney's burst esophagus? Or Shelly's almost-death from heatstroke? Or how about the fact that April has an evil, personalty-bending mutant demon inside her body that's slowly killing her from the inside?!
In The One, Beth's friends purposely ditch her because they think she's lying about being sick.
In the first book of his autobiography, Marcel Pagnol says "I think that man is cruel by nature. Children prove it everyday."
Some of the career kids in The Hunger Games clearly take pleasure in the Games and it's implied that the careers in the 74th Games torture one of the other tributes. At first they are just presented as cruel but by and by the story begins to discuss whether they are cruel by nature or if they have been conditioned to behave that way.
The last line of Peter Pan: "And so it will go on as long as children are gay and innocent and heartless."
This seems to be the main point of Sergey Lukyanenko's novel The Knights Of Forty Islands, where teens are kidnapped and placed on small islands connected by bridges (each island is connected to three others). They find out that whichever island manages to conquer all others gets to go home. It's not much of a surprise to learn that The Cake Is a Lie, all the kids are copies of the originals; the aliens never intend to release them. Each island has about half-a-dozen kids (both male and female), and battles normally take place on these narrow bridges using wooden swords that turn to metal with fueled by anger. The protagonist comes up with a plan to create an Alliance of several islands that either gets other islands to join or conquers them. However, the Alliance falls apart due to this trope. In fact, this isn't the first time such a scheme failed. The author planned to write a sequel, but got sidetracked, only having written one chapter.