The trouble with most of the responses advocating this is that they come from the perspective of someone who is an adult, and with time to formulate in their mind what they would do. Same doesn't hold true in the moment. Especially with kids and young adults who are likely to be terrified and not thinking clearly. It takes intensive and repetitive training to know how to act under pressure.

Same for those that claim deterrence. The presupposes that the nut is actually going to think through the act in a logical and sane way. People that believe this are the same people that think that showing a gun will deter a criminal. It is a fallacy. If you are going to pull a gun for deterence, then you should be prepared, and intend, to use that gun. Waving it around will likely get it taken from you and used on you.

Teach your children to run away. The moment they sense danger, they should run, en mass, for the nearest exit. A nut is going to do what they are going to do. Rather than teach your children to take them on, improve security at the school, improve training for the teachers, stop bullying, perhaps a program of arming certain employees ala the Sky Cops (but only with intensive training).