The vampire hunters could be the main antagonists. They're always on the lookout for vampires, and in a high school setting, they might suspect the male vampire and try to expose or destroy him.
In some cases, a jealous human student could be an antagonist. If the male vampire is popular or has the attention of a girl the human likes, the human might start to suspect something strange about him and try to uncover his secret, causing all sorts of problems for the male vampire in high school.
Another possible antagonist could be a powerful vampire from an older generation. Maybe this vampire has different ideas about how the male vampire should use his powers. He could be trying to push the male vampire into a more dark and violent way of life, which goes against what the male vampire wants for himself in high school.
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.
The main characters could be a vampire girl who is the popular one in school but has to hide her true nature. And a human boy who falls in love with her not knowing she's a vampire.
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 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.
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.
The main characters could be a shy human girl who is an outcast in high school and a mysterious vampire boy. The girl might be the type who is always reading books in the corner of the library and the boy could be the new student who seems to have a secret.
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.
The Soul Reapers could be antagonists if they view Ichigo's vampirism as a threat to the balance between the living and the dead. They might try to hunt him down to eliminate this new, unregulated force.
In this fanfiction, the Church or religious authorities in Camelot could be the main antagonists. Vampires are often associated with the devil or evil in religious beliefs. So, they might see Merlin's transformation into a vampire as a sign of him being corrupted by dark forces. They would try to exorcise or destroy him, not understanding that he's still the same Merlin at heart, just with a different form and new powers.