He'd probably teach Greek mythology for sure. Given his personal experiences with the gods and monsters from Greek mythology, he could provide a very different perspective compared to other professors. He might also teach some sort of adventure or survival skills class, sharing his knowledge from all the quests he's been on.
I think Percy would be great at teaching a self - defense class. His combat skills, learned from fighting monsters, would be valuable knowledge to pass on. And he could also teach a class on environmental studies, especially when it comes to water ecosystems. He could use his powers to show students the importance of clean water and how to protect water sources in a very practical and engaging way.