As I have often written, the only measure that will not only deter killers, but may stop them before they can injure or kill anyone is arming as many willing staff members as possible with concealed handguns. Knowing that any teacher in any building may very well be armed is the only predictably effective factor that could cause a killer to choose a softer target, or to abandon a murderous plot entirely.
Ironically, it is this factor that will keep most school districts from taking this step. Once the process of arming staff begins, any school district refusing to do the same paints a large target on the backs of their children and teachers. If the Smithville district arms teachers, but the next-door Jonestown district does not, killers will simply choose the gun free schools of Jonestown. As poorly informed as many school boards and administrators are on security and firearm issues, they understand this. As horrifying as it sounds, a great many of them would prefer to keep their schools victim disarmament, free fire zones than actually deter killers and save lives. Maintaining the ideological purity of progressive narratives takes precedence. Feeling safe is much more important than being safe.
Sickening to think such a thing true, but I think Mike is right. Yes, I know there are some idealists who just cannot stomach that children can only be protected by people with firearms. But, idealism, no matter how sincere or heartfelt, loses every time it meets reality.