I think Percy's classmates could be quite affected too. Their concentration gets broken, and if they were in the middle of an important lesson or test, it could really annoy them. Some might also be worried that the teacher will be in a bad mood for the rest of the class because of Percy's phone, which could affect how the teacher interacts with them for the remainder of the period.
Maybe Percy gets in trouble with the teacher. Teachers usually don't like phones going off during class.
Well, he might get in trouble with the teacher. Teachers usually don't like phones ringing during class. The teacher could confiscate his phone or give him a detention.
The Akatsuki would be highly affected. Since Naruto has future knowledge, he could stop their plans before they even start. Also, his friends like Sasuke would be affected as Naruto could prevent Sasuke from going down the wrong path.
Maybe a new character who is a half - nymph. She has a special connection with nature and can make plants grow in the middle of the gymnasium during class. This would make for some really interesting scenarios in the gym class. For example, she could create a maze of vines for a race. Or there could be a character who is a demigod from a very minor god that we haven't heard much about. He has a unique power that he discovers during gym class, like being able to turn invisible for short periods of time which gives him an edge in certain activities.
His relationship with Annabeth could be strained. She might be initially afraid of the changes in Percy due to his demon half. However, over time, as she sees that he's still the same Percy at heart, they could work through it and their relationship could become even stronger as they face this new challenge together.
His best friends, Ron and Hermione, would probably be the most affected. They're used to seeing him with his old hair, so the change might be a shock at first.
Probably Annabeth. She's always full of knowledge and strategies.
Annabeth could be a main character. She might be desperately trying to find Percy after he jumps off Olympus. She could use her wisdom and strategic skills to track him down, facing various obstacles along the way, like mythical beasts or other unfriendly demigods. Another character could be Grover. He could sense Percy's distress and join Annabeth in the search, using his nature - related powers to help them.
Obviously, Percy would be one of the main characters. And probably some of his classmates who are with him on the ship that gets wrecked.
Another option for the main antagonist could be a spirit of an ancient Greek scholar who was banished from the world of the gods. This spirit haunts the Greek class and tries to possess Percy because he sees Percy as a way to get revenge on the gods. It causes chaos in the class, making it difficult for Percy to focus on his studies and also putting his friends at risk.