A strict teacher could be an antagonist. Maybe this teacher has old - fashioned views and doesn't like Percy's unorthodox ways or Artemis's independent streak. The teacher could constantly give them detention or try to stop them from participating in certain school events that are important for the plot.
Gary could be an antagonist. He might be jealous of Ash's relationship with Dawn and try to cause trouble between them. Maybe he spreads rumors about Ash in the school. Another possibility is a popular student who doesn't like that Ash and Dawn are getting all the attention during a school event and tries to sabotage them.
The other gods might be antagonists. For example, Zeus could be against Artemis getting Percy back because he doesn't trust Percy or he has his own plans for Percy that don't align with Artemis'. Another god like Ares could also be an antagonist as he might see Percy as a threat and not want him to be on Artemis' side.
Perhaps there's a new villain, a powerful sorcerer who has a vendetta against Percy. The sorcerer discovers that Percy is now at high school and decides to target the school. He uses his magic to create illusions and manipulate the minds of the students and faculty, turning them against Percy. The sorcerer might also be after the secrets Percy learned in Tartarus, thinking it can give him more power.
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.
The school bully could be a main antagonist. He might pick on Austin for being a singer and think he's too full of himself, or he could target Ally for being a bit of a nerd. Another could be a jealous rival singer in the school who wants to take Austin's place and will do anything to undermine him. And there could also be a strict teacher who doesn't believe in their music dreams and tries to stop them from performing at school events.
Perhaps the antagonist could be a teacher who has a dark secret. This teacher might be using the students for his or her own gain. Sasuke and Sakura notice some strange behavior from this teacher and start to suspect that they are up to no good. They then become the targets of this teacher's wrath as they get closer to uncovering the truth.
The Titans could be the main antagonists. Even though they have been defeated before, some of them might have found a way to return or their followers could be causing trouble. They see Artemis as a threat or a target for their revenge against the gods. Another possibility is a new group of powerful monsters that have banded together and are specifically targeting Artemis for her power and influence among the demigods.
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.
Often, it could be a jealous classmate. Maybe someone who has a crush on Percy and sees Annabeth as a threat, so they try to create problems between them. Another could be a strict teacher who doesn't like Percy's carefree attitude and Annabeth's independent thinking, always trying to give them a hard time.
Obviously, Percy and Artemis are the main characters. Percy, a demigod with Poseidon's powers, often brings his adventurous and sometimes reckless nature into the high school setting. Artemis, the goddess of the hunt, is strong - willed and independent. Their interactions are what drive most of these fanfiction stories.