“Spurred in part by the Columbine and Virginia Tech shootings, many school districts around the country adopted zero-tolerance policies on the possession of weapons on school grounds.”
Which is what those two campuses had, and the CAUSE of so many deaths!
Yes, that was just sloppy writing — zero-tolerance policies have been around for years, and contribute to the problems at Columbine and VT, first because of the anti-gun rules which keep people from defending themselves, but moreso that the zero-tolerance policies keep children from learning how to behave themselves, so when they get into less structured environments they easily act out in violent ways.