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.
One idea could be Percy trying to hide his demigod powers from his high school classmates while still using them to solve normal high school problems like bullying. Another could be Percy forming a secret club in school for other kids with special abilities. Or maybe Percy getting into a friendly rivalry with the school's star athlete in a non - combat - related sport like track and field.
Percy could meet the school bullies. And they might pick on him at first, but then be in for a big surprise when he shows his strength. Also, he might encounter some teachers who suspect there's something different about him. Maybe a science teacher who notices his quick reflexes during an experiment or a history teacher who is amazed by his knowledge of ancient mythology.
In the fanfiction, when Annabeth goes to Percy's high school, she would probably form a study group. With her knowledge, she'd help Percy and his friends get better grades. Also, she might start some kind of architecture or history club in the school since she has a passion for those things.
Maybe Percy would struggle to fit in at first. He's used to a different world, like Camp Half - Blood. But with Annabeth by his side, he could start to adapt. He might make new friends or get into some trouble with the school bullies, only to show off his demigod skills in the end.
He might meet normal high school students. There could be the typical jocks, nerds, and popular kids. Maybe he'd encounter a really smart student who is intrigued by Percy's different way of thinking. Or he could meet some mean students who pick on him at first but then learn to respect him.
There could be a school janitor who is actually an old demigod who retired from the hero business. He could give Percy some valuable advice based on his own experiences. And of course, Percy will surely meet Annabeth's classmates, some of whom could be friendly and some not so much, depending on how they react to the new boy in school.
Maybe they are new students at the same high school. Percy, being the son of Poseidon, has some strange things happen around him. Jason, from his own background, tries to figure out what's going on with Percy and they end up becoming friends.