The Secret Place, by Tana French

Tana French changes things up so completely you can never know what to expect from her.  The Secret Place is part police procedural, part coming of age (?) story.  A young man is bludgeoned to death on the grounds of a fancy girls' school.  A year later, one of the boarders at the school instigates a reopening of the case in a day-long investigation conducted by Antoinette Conway and Stephen Moran of Chris Harper's death.  Intermittently, flashbacks explore the friendship between Holly, Becca, Julia and Selena, and the threat to that friendship posed by Chris Harper.