A student walks down a Harding High hallway wearing headphones, chanting along to violent rap lyrics. Teacher Erik Brandt taps him on the shoulder. Turn it down, he gestures. The kid stares at Brandt with chilling intensity. He points at the older man, fingers bent in the shape of a gun, and shoots. Then moves on. Within Harding's corridors is a turbulent clutter of students who push and cuss and bully their way from one end of the building to another. Brandt, a finalist for Minnesota's Teacher of the Year and a 20-year veteran of the English department, doubles as...