AFTER THE MASSACRE at Colorado's Columbine High School in 1999, police in Prince William County wisely devised an emergency plan to respond to gun threats in schools. So when a camouflage-clad 12-year-old appeared in Bull Run Middle School last Friday armed with two rifles and a shotgun, the police were ready, arriving on the scene swiftly, sweeping into the school and arresting the boy. Cool-headed teachers and staff alerted police, hustled students away from danger and kept their wits at a perilous moment.

But the chilling fact remains that at least 10 minutes elapsed before the boy, who pointed a weapon at students, parents and school staff, was taken into custody -- 10 minutes in which he might have done unspeakable damage. No one should suppose his act was an aberration; this year alone there have been scores of incidents across the country in which high school students, and even grade school pupils, entered schools bearing firearms, often loaded. It is cold comfort that few of those students opened fire.

There are no reliable figures, but it's a safe guess that in many or most of these instances, the guns were owned by the students' parents. That was the case with the boy at Bull Run Middle School. According to Prince William Commonwealth's Attorney Paul B. Ebert, the weapons belonged to the boy's father, who kept them unloaded -- but apparently unsecured -- in a rack at home; the boy also managed to find ammunition at home. The boy's mother, a cafeteria worker at the school, was charged with possession of a weapon on school property. She gave her son a ride to school and noticed the guns in her van but apparently thought so little of it that she locked the vehicle and never made a report to police. The boy went back with a key and retrieved the guns, police said.

In this case, it seems plain the parents' approach to firearm safety was cavalier. A broader concern is the laxity of laws that might induce parents to exercise common-sense responsibility with their guns. Even if the guns used by the boy at Bull Run had been loaded and left within his reach at home, under Virginia law the father could have been charged with only a Class 3 misdemeanor, which carries a $500 fine. In other states, parents can be charged with a crime only if their children injure or kill someone with their parents' weapons. And while most gun manufacturers provide a trigger lock with their firearms, there is nothing to compel gun owners to use them.

The National Rifle Association advises gun owners to store their firearms locked away when not in use, but it has opposed mandatory locking laws. The group, which wields considerable power in Virginia's legislature and beyond, argues that guns may be "in use" -- and therefore left unsecured -- if owners think they need them for protection. Police and prosecutors are left to lament the absence of muscular laws requiring gun owners to keep their weapons out of the hands of children. So are we all.