The problem is that if you put guards in schools, the killers go to malls, you put guards is malls, the killers go to theaters, or political rallys, etc, etc. They're always going to "hit us where we ain't".
Having said that, what's wrong with guards in schools, in body armor, armed with tazers and trank guns. Hit an innocent bystander, nobody dies.
As for the "What is an assault weapon?" Aside from "any gun aimed at you", it can not be defined by stock barrel, clip or any cosmetic consideration, it must be defined by capability.