Just because you train someone and give them a gun does not necessarily enable them to prevent a situation such as what happened. It goes back to the flight and fight scenario. Then once the decision is made, is the person proficient enough to disable the threat? Has the person been thru the proper stress inoculation of simulated return fire? Its one thing to shoot a paper target and another to shoot at something looking and aimed in on you.

Probably be a lot cheaper to just have a metal detector and control access in and out of schools than to risk more damage from some one trying to curve a bullet around two third graders.