The high school principal who has very strict and old - fashioned views could be an antagonist. He might not approve of Belle and Adam spending time together, thinking it's a distraction from their studies. He could try to enforce rules that keep them apart, like not allowing them to be partners in any school activities or even giving them detention when they are seen together.
A jealous classmate could also be an antagonist. Maybe someone who has a crush on Adam and doesn't like that he's starting to show an interest in Belle. This person could try to sabotage Belle's reputation at school by making up lies about her or trying to get her in trouble with the teachers, all in an attempt to drive a wedge between Belle and Adam.
In this kind of fanfiction, Gaara could start as an antagonist. He's the new kid from another school who transfers in. He has a very cold and intimidating demeanor. He gets into fights with Naruto and his friends because he has his own ideas about how things should be in the school. He doesn't trust easily and thinks everyone is out to get him. But as the story progresses, he could turn into an ally. Also, Danzo could be an antagonist. He's a strict, old - fashioned teacher who doesn't like Naruto's unruly behavior and tries to expel him from school at every chance he gets.
One possible antagonist could be a popular girl in school who is jealous of Sakura's popularity and tries to undermine her at every turn. For example, she might spread false rumors about Sakura or try to steal her friends. Another could be a strict teacher who doesn't believe in Sakura's abilities and is always giving her a hard time, like constantly doubting her work in class or not allowing her to participate in certain school activities.
Alois could be a main antagonist. In a high school setting, he might be the jealous rival of Ciel. He could try to undermine Ciel's position as student council president or spread rumors about him. His butler, Claude, would be helping him with his devious plans.
Perhaps there's a new villain, a powerful sorcerer who has a vendetta against Percy. The sorcerer discovers that Percy is now at high school and decides to target the school. He uses his magic to create illusions and manipulate the minds of the students and faculty, turning them against Percy. The sorcerer might also be after the secrets Percy learned in Tartarus, thinking it can give him more power.
Often, it could be a jealous classmate. Maybe someone who has a crush on Percy and sees Annabeth as a threat, so they try to create problems between them. Another could be a strict teacher who doesn't like Percy's carefree attitude and Annabeth's independent thinking, always trying to give them a hard time.
The school bully could be a main antagonist. He might pick on Austin for being a singer and think he's too full of himself, or he could target Ally for being a bit of a nerd. Another could be a jealous rival singer in the school who wants to take Austin's place and will do anything to undermine him. And there could also be a strict teacher who doesn't believe in their music dreams and tries to stop them from performing at school events.
One idea could be Belle being the new student at Adam's high school. Adam is the popular but brooding student. Belle's intelligence and kindness start to break through Adam's tough exterior as they get paired for a school project. This could lead to a slow - burn romance filled with typical high school drama like jealous classmates and misunderstandings.
Another possible antagonist could be Zelena. Jealous of Belle's happiness and the new life she's bringing into the world, Zelena could try to cause trouble. She might use her magic to create false scares or difficulties during the pregnancy, like making Belle believe there are problems with the baby when there really aren't.
There could be a group of mean students who don't like Naruto because he's different. They constantly bully him and try to keep him away from Sakura. They might be the typical 'popular' kids who think Naruto doesn't belong in their school or around Sakura.
Often, in fanfiction, there can be dark witches or warlocks who oppose Adam and Cassie. They might be jealous of their powers or have some vendetta against the Secret Circle as a whole.