The Lost Hero by Rick Riordan

The Lost Hero is a 2010 fantasy-adventure novel written by Rick Riordan and is based on Greek and Roman mythology. The Lost Hero is the first book in the series The Heroes of Olympus, the next series about Camp Half-Blood. It was preceded by the Percy Jackson & the Olympians series, which focused solely on Greek mythology.
As the book opens, Jason awakens on a school bus, unable to remember who or where he is, or anything about his past. He is sitting next to Piper McLean and Leo Valdez, who call him by name and say they are his girlfriend and best friend respectively. All three are part of a class field trip to the Grand Canyon, and after they arrive, storm spirits attack the trio and their trip leader, Coach Gleeson Hedge. Jason surprises everyone when one of his coins turns into a sword. Coach Hedge, who reveals himself to be a satyr during the fight, is taken captive by a fleeing spirit. After the battle, a flying chariot arrives to rescue the trio, but one of the people in it, Annabeth, is upset when she discovers that her missing boyfriend, Percy Jackson, is not there as she expected. Annabeth, seeking Percy, was told in a vision from the goddess Hera to look there for the "guy with one shoe", but this turns out to be Jason, who had a shoe destroyed during the fight.
Jason, Piper and Leo are told that they are demigods and are taken back to Camp Half-Blood where they meet other demigod children like themselves. There, Leo is revealed as a son of Hephaestus, Piper as a daughter of Aphrodite and Jason as a son of Zeus ... (Summary by Wikipedia)