to pull back from the abyss for a moment: the guns were all owned by the mother. a kindergarten teacher who owns an assault rifle and two 9s? that seems odd.
the way they're securing these schools (and all these strategies will now certainly be redoubled) also converts them into traps. they were leading kids out the front door past the (dead) killer and the horror. a fortress makes you feel secure until you can't get out.