Interesting political ideas, but pretty much irrelevant to preventing a Beslan or Mumbai style attack.
A van (or several) pulls into a school parking lot and half a dozen to several dozen heavily armed terrorists enter the building.
Having hundreds or thousands of armed citizens a few minutes away does little good. By the time they arrive the terrorists have the children under control. Hundreds of hostages.
The only thing that CAN stop these guys is an equivalent or greater force in the right place at the right time. Which isn’t at all likely unless you have relevant intelligence, in which case of course they’d normally be stopped before they get anywhere near the school.
Armed teachers is the only possibility I can think of, but of course most schools in America are “gun-free” zones and most teachers are anti-gun. Certainly their unions are. And such an assault wouldn’t be stopped by a few handguns, in any event.
What are the Israelis doing to stop attacks on their schools? I don’t know. Whatever it is, we need to copy that.