Last to Die (Rizzoli & Isles #10) by Tess Gerritsen

Three children are brought together by seemingly motiveless and extreme acts of violence. Orphaned and alone, they are taken in as students at Evensong, a boarding school for emotionally traumatized children in the remote Maine wilderness.

Forensic pathologist Maura Isles already has a connection with the school – Julian ‘Rat’ Perkins, the 16-year-old boy she met during a previous case, is now living there. But she suspects that the Evensong founders may be using the school for their own agenda. And her concerns grow when Detective Jane Rizzoli is asked to investigate yet another attempt on the life of one of the orphans at the school . . .