The Screaming Season by Nancy Holder

The gutsy heroine of Possessions and The Evil Within returns for another year of boarding school at the haunted Marlwood Academy. Lindsay wakes to find herself strapped down in the infirmary. She had a breakdown and might have tried to kill her nemesis Mandy or Mandy's boyfriend, Troy-or both. The details are hazy, but one thing is certain: she is possessed by a spirit she cannot trust.
Lindsay soon realizes that nowhere on campus is safe. Then, she finds a surprising ally in her former rival. Together, Lindsay and Mandy must figure out who can be trusted-and who wants them dead. But when Lindsay's ex-boyfriend shows up at Marlwood, she is given a chance to get away and be free of the curse. Will she take Riley's hand and run, or team up with a new love to save Marlwood from the evil spirits forever?