It could be another vampire who is jealous of Bella. Maybe this vampire has been at the school for a while and doesn't like the attention Bella is getting. So, they try to create problems for her, like spreading rumors about her or trying to get her into trouble with the school authorities.
Perhaps the main antagonist is a powerful witch at the school. Witches and vampires often have a complicated relationship in supernatural stories. The witch could sense Bella's power and see her as a threat to the balance of the supernatural elements in the school, so she sets out to oppose Bella at every turn.
Well, in this fanfiction, Bella might face challenges in hiding her vampire nature at school. For example, she could struggle with controlling her super strength during gym class or her enhanced senses in a crowded cafeteria. Maybe she'll have to deal with other students who are suspicious of her new, more alluring appearance.
A powerful sorcerer could be the main antagonist. He might be after Percy's powers or trying to use the school as a base to spread chaos. Maybe he wants to open a portal to the underworld right in the middle of the schoolyard.
It could be a new teacher who is secretly a minor villain from the Percy Jackson universe. This teacher might be trying to undermine Percy's confidence and expose his demigod side to the mortal world. They could set up difficult situations for Percy in class, like impossible tests or false accusations of cheating. Also, they might try to turn Percy's friends against him by spreading rumors. The teacher could be working for a bigger bad guy in the background, but their main goal is to make Percy's high school life miserable.
Another option for the main antagonist in the fanfiction could be a jealous classmate. This classmate might be envious of Spider Man's popularity and powers. He or she could try to sabotage Spider Man at school, like spreading rumors about him or trying to get him in trouble with the principal. This would make Spider Man's school life even more difficult as he has to deal with both this classmate and any real threats to the city.
It could be a vampire hunter who has been tracking Bella's family for generations since finding out about her great grandfather. This hunter might see Bella as a way to finally get to the root of the vampire bloodline in her family.
In Twilight fanfiction, Vampire Bella going to school has multiple difficulties. Firstly, her appearance. Even though she tries to blend in, her pale skin and other vampiric features might make her stand out. Secondly, her academic performance. With her new vampire - enhanced mind, she may find it hard to not seem too smart and draw unwanted attention. Thirdly, the relationships. She has to keep her distance from her old human friends in a way that doesn't seem too odd, while also dealing with any new vampire - related relationships that might form at school.
Tom Riddle himself could be the main antagonist. Even in his school days, he was showing signs of his future as Voldemort. His dark thoughts and actions towards other students or his exploration of dark magic could put him in direct opposition to Harry.
She becomes more cautious. She has to watch every move she makes to avoid revealing her true nature.
It could be another powerful enchantress who is jealous of Bella's powers and wants to steal them for herself. She might constantly try to set traps for Bella and undermine her at every turn.
Maybe there's an evil sorcerer or wizard who sees Bella's weakness as an opportunity. He or she might want to use Bella's weakness to gain something, like access to a powerful artifact that only Bella can reach because of her limitations. The sorcerer would be constantly trying to manipulate her into doing his bidding.