The Bronze Key by Cassandra Clare & Holly Black

Magic can save you. Magic can kill you.

Students at the Magisterium are supposed to be safe. Under the watchful eyes of the mages, they are taught to use magic to bring order to a chaotic world.

But now the chaos is fighting back. Call, Tamara, and Aaron should be worrying about things like pop quizzes and magic contests. Instead, after the shocking death of one of their classmates, they must track down a sinister killer… and risk their own lives in the process.

As Call, Tamara, and Aaron discover, magic can only be as good as the person who wields it. In evil hands, it has the capacity to do immeasurable harm, unless it is stopped in time.