The Perilous Sea (The Elemental Trilogy #2) by Sherry Thomas

After spending the summer away from each other, Titus and Iolanthe (still disguised as Archer Fairfax) are eager to return to Eton College to resume their training to fight the Bane. Although no longer bound to Titus by a blood oath, Iolanthe is more committed than ever to fulfilling her destiny—especially with the agents of Atlantis quickly closing in.

Soon after arriving at school, though, Titus makes a shocking discovery, one that makes him question everything he previously believed about their mission. Faced with this devastating realization, Iolanthe is forced to come to terms with her new role, while Titus must choose between following his mother's prophecies—and forging a divergent path to an unknowable future.