Zero tolerance is the hammer that makes violence prevention programs effective, said Elliott Phelps, chief of school police for the Clark County School District. "If you don't have the hammer, your prevention programs are taken with a grain of salt," Phelps said. "There has to be both prevention and consequence, and there has to be a balance between the two." Like most education officials across the nation, Clark County school leaders have struggled to find that balance. In one extreme, students who pose a potential threat to others by carrying or concealing weapons while on school property face immediate expulsion...