Wizards of the Mound by Ray Clifford Martinez II

Like his mentor before him, Lazlo is a teacher. And not just any instructor: Lazlo takes the problem students; the kids nobody else at the Mound will risk teaching. He’s in the perfect position to do that because even though he works for the academy, Lazlo refuses to set foot on campus again. Not after what happened last time.

Unfortunately, Lazlo’s vows go out the window when his old mentor dies in a mysterious dueling accident. Forced to return to campus, Lazlo quickly discovers evidence that his mentor was murdered. But none of the other masters believe him- in fact, no one is acting quite like themselves.

Lazlo must solve his mentor’s murder himself- with the help of his star pupil, an out-of-place druid, and a former friend who Lazlo once betrayed. He’s made a lot of mistakes, but Lazlo is determined to see justice done this time.