You are forgetting about the kids growing up and realizing their workplace and homes and public places they frequent don't have the super necessary bulletproof safety sheds.

This idea that we should turn schools into fortresses always ignores the side effect that you are telling children they aren't safe unless there are draconian and in some cases ridiculous security measures in place to prevent or mitigate events that odds are would never have happened at their school in the first place.

These kids are going to be afraid to exist in the real world.