https://onlinereadfreenovel.com/rick-riordan/2665-the_lost_hero.html

Here's the blurb:

Jason has a problem. He doesn't remember anything before waking up on a school bus holding hands with a girl. Apparently she's his girlfriend Piper, his best friend is a kid named Leo, and they're all students in the Wilderness School, a boarding school for "bad kids." What he did to end up here, Jason has no idea-except that everything seems very wrong.