Note that in the Pearl High School shooting (item #1), Assistant Principal Joel Myrick had to retrieve his gun from his truck parked off campus because it had to stay outside the "gun-free school zone". It's unclear how many of the 9 victims could have been avoided but for the "gun free school zone."
Ok, so there are some cases where it helped stop the attack. Going a step further, how many of those had AKs and maybe bullet proof vests? Should everyone have armor piercing bullets too?
How many of the attacks happened because it was easy to get guns in the first place?
A terrorist will always be able to procure guns. But what about an unhappy employee, an angry teenager ?