In a 'percabeth meet in high school' fanfiction, they could meet in detention. Percy got in trouble for getting into a fight with a guy who was picking on a younger student. Annabeth is there because she was caught arguing with a teacher about a wrong grade. At first, they don't get along. Percy thinks Annabeth is a know - it - all, and Annabeth thinks Percy is too impulsive. But as they spend more time in detention together, they start to understand each other. They bond over their shared love for Greek mythology that they discover they both have. Eventually, they start dating and support each other through high school.
Maybe Percy is on the swim team and Annabeth is tutoring some students in the library near the pool. Percy often sees Annabeth when he's coming in and out of practice. One day, he has to use the library for a research paper and asks Annabeth for help. She initially thinks he's just a jock, but as they work together, she discovers his hidden intelligence and his kind heart. Meanwhile, Percy is attracted to her confidence and her love for knowledge. Their relationship blossoms as they help each other through high school dramas, like getting ready for prom and dealing with college applications.
One possible plot could be that Percy and Annabeth are in the same high school but in different social circles at first. Percy is the new kid, a bit of a loner who loves the oceanography class. Annabeth is the smart, popular girl in the architecture club. They get paired for a project on building a model of an ancient Greek structure near the sea. Through this project, they start to notice each other's good qualities, like Percy's determination and Annabeth's intelligence. They gradually become friends and then realize they have deeper feelings as they face challenges together, like dealing with a bully who tries to destroy their project.