In the 2009-2010 year, 17 children ages 5 to 18 died in homicides at school, traveling to or from campuses or at school events, according to the U.S. Education Department. That’s about half the annual figure during much of the 1990s. In a population of more than 50 million students, the school-related death toll has been about 1 percent of all homicides for that age group.

Responding to high-profile shootings since the late 1980s, school districts stationed police officers on campuses, established single points of entry for buildings, trained staff through “lockdown drills” to minimize casualties and promoted programs to teach conflict resolution and fight bullying.

“Schools are safe, and they are safer than they were,” said Stephen Brock, a professor of school psychology at California State University, Sacramento, who has trained more than 4,000 educators in preventing and responding to such crises. “Unless we turn our schools into prisons, we aren’t going to be able to prevent some truly motivated person from entering school grounds. There are things we should do, but there are limits.”