Bear spray works
In all of the reactive media coverage, I have not seen or heard anyone suggest a very simple, effective and nonlethal preventive measure for use by those with large concentrations of people under their charge (such as school principals).
Bear spray, or its human equivalent in a Mace-type product, will totally incapacitate an assailant from as far away as 30 feet, does not need to be aimed precisely and will not seriously injure an innocent bystander.
If I were placed in charge of a school system, one of my first directives would place this type of product in appropriate hands in every school, along with the training on how and when to use it.
It would be ludicrous, as some are now suggesting, to arm officials such as principals with firearms, but it is equally ludicrous to arm them with nothing.