You aren't "asking" them to do anything. They would be VOLUNTEERING to do it. If I were teaching, I would volunteer in a heartbeat.
The thing is, most teachers won’t volunteer. And of those few who do, are they capable of acting as the police did in that video?
And how are the few volunteer teachers supposed to do their active shooter routine, lock classroom door, get kids against the wall on same side as door, or whatever is called for, and also go out and protect the rest of the school?