Frost by Marianna Baer

Leena Thomas's senior year at boarding school begins with a shock: Frost House, her cozy dorm of close friends, has been assigned an unexpected roommate -- confrontational, eccentric Celeste Lazar. But while Leena's anxiety about a threat to her sanctuary proves valid, it becomes less and less clear whether the threat lies with her new roommate, within Leena's own mind, or within the very nature of Frost House itself. Mysterious happenings in the dorm, an intense triangle between Leena, Celeste, and Celeste's brother, and the reawakening of childhood fears, all push Leena to take increasingly desperate measures to feel safe. FROST is the story of a haunting. As to whether the demons are supernatural or psychological...well, which answer would let you sleep at night?