If you CAN.
Shooting at an armed combatant is a whole lot different from shooting unarmed kids...particularly when he KNOWS he's a target.
He doesn’t know he is a target. That killer with his own AR shoots him from concealment and walks into the school. You have to have at least two of these guys on campus and that is getting even more expensive. Why cannot even Freepers accept the obviouis and probably only sensible and certainly the cheapest solution- allowing teachers and staff to carry?