Some are calling for an armed guard in every school, but Columbine HAD one and that did not stop the murders. A guard cannot be every place in the school at once, any more than cops can be everywhere in the community.
So why not try a very simple practice at every school in every community? That is, simply lock every classroom door, all the time -- not just when the teacher gets a warning or hears shots, but ALL the time. Kids going to the bathroom could have a key as well as a hall pass. If the principal wants to drop by, he has the intercom to inform the teacher. And even when he knocks on the door, a simple "Who is it?" would assure that it was safe to open the door and would teach the kids a lesson about opening doors at home.