Teachers don't have to be trained to SWAT levels. They probably will not be hunter-killer teams, but room defense leaders. Schools go into lockdown, kids huddle in a corner ready to throw books, staplers, etc if the steel door is breached. Teacher could get a 20 gauge pump gun from a locker in the room, fort up in a good spot and defend the doorway with buckshot if needed. But then, some are easily discouraged and knowing there is buckshot somewhere, might think twice.