It could be a jealous classmate. Maybe someone who has a crush on Armin and sees Annie as a threat to their relationship. This person could try to spread rumors about Annie to make Armin stay away from her.
A strict teacher who doesn't like the idea of Armin and Annie spending so much time together. The teacher could constantly give them extra work or try to separate them during school activities, believing that their friendship is a distraction from their studies.
Loki could be a great main antagonist. He could be the mischievous student who's always causing trouble for the Avengers. He might use his magic to play pranks on them or steal their homework to make them look bad in front of the teachers.
Perhaps a strict teacher who doesn't like Naruto's unorthodox ways and Anko's rebellious attitude. The teacher might try to split them up and prevent them from participating in school activities together. For example, if they want to enter a school competition as a team, the teacher could create obstacles.
Perhaps a jealous classmate who discovers Artemis's connection to the fairy world. This classmate wants to use that knowledge to gain power and popularity, so they start causing trouble for Artemis and his friends, trying to expose them to the wrong people or get in the way of their fairy - related investigations.
Perhaps Orochimaru from Naruto could be the main antagonist. He's always seeking more power, and in this fanfic, he might see the powers in 'The God of High School' as a means to achieve ultimate power. He could try to manipulate the events of the 'The God of High School' tournaments to his advantage, all while causing chaos in Naruto's world as well.
Gary could be a main antagonist. He might be jealous of Ash's popularity at school and constantly try to undermine him in Pokemon battles and in front of their classmates.
It could be the school principal. The girl wants to put on a more edgy and progressive play that challenges some of the school's traditional values. The principal, being more conservative, tries to stop her from doing so. He or she makes it difficult for her to get the necessary resources and permissions, becoming the main obstacle in her acting journey in high school.
Perhaps a former teacher who always had it in for Pete. At the reunion, this teacher could be there as a guest and start criticizing Pete's life choices or bring up old mistakes he made in school. The teacher might be jealous of Pete's new - found confidence or success and wants to put him down.
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.
Maybe there could be a plot where Annie is a new student at the high school. Armin is assigned to show her around. At first, Annie is cold and distant, but as Armin shares stories about the school and its traditions, she starts to open up and they form an unexpected friendship that could potentially turn into something more.
It could be a new demigod who has turned to the dark side. Jealous of Percy's popularity and Artemis' power, this demigod could try to create chaos in the high school and turn students against them.