I hate to say it, but it's not as outrageous as it sounds to us and it might just help to cut the number of attacks. We're talking about America, not here. The right to bear arms is deep in the American psyche, whether we think it should be or not. Armed staff may be able to disable/kill an attacker before many children are killed. This has got to be a benefit.
The other benefit would be the deterrent. Real nutcases won't be stopped, but grudge-bearing immature young adults might well be. Again the result is lives saved.
My own feeling is, given that Americans will never be persuaded to give up carrying guns (draconian legislation would only lead to underground trade), that they should be free to buy shotguns and non-automatic pistols. Other arms would need a licensing system.
It might cut the death toll.