Twenty years after two students killed 12 classmates and a teacher at Columbine High School in Colorado, lawmakers and safety experts continue to debate how to secure the nation's K-12 schools against gun violence. Recent mass shootings have heightened the issue's urgency, including two last year — at high schools in Parkland, Fla., and Santa Fe, Texas, that took a combined 27 lives — and this month's shooting at a Denver-area charter school that killed one and wounded eight. Congress continues to resist new gun laws, and some of the steps states and localities are taking to protect students are deeply controversial. Most states now allow armed school security personnel, and some districts are arming teachers, a move that has divided parents, teachers and safety experts. At the same time, active-shooter drills that expose young children to the possibility of violence also are sparking debate. Meanwhile, critics complain that while millions of dollars are being spent to “harden” schools with cameras, metal detectors and other paraphernalia, mental health services that might keep troubled students from committing violence remain underfunded.