All other things being equal, a hostile element will always try to bring a vector of force necessary to beat a target's defenses. Up-gun defenses and the likely result is the hostile element up-guns themselves or shifts tactics to balance the change. The end result is the same confrontation takes place just with greater energy involved which generally is bad for innocents in the line of fire. Essentially, an arms race.
The only legitimate solution is to engage the hostile element before they become one or before they engage their target. It is probably much more useful and effective to talk about social solutions to these issues.
And if you really want to harden schools, hardening their interiors isn't really the way to go. Once a hostile element has gotten inside a school, there is a very good chance bad things are going to happen regardless of an armed presence. Hardening really should take place at the perimeter. Of course the NRA doesn't sell fences so you won't hear them taking such a stance.