Notice they never try to shoot up a police or ICE HQ.

It's not so much that children can't shoot back (a three-year-old can pull a handgun trigger.), it's because neither the students nor the teachers are armed. And most of the teachers don't want to be armed either.

There are a few issues, there will always be idiots that feel safe because nothing has happened to them so by default nothing ever will, they only wake up when the pain comes to them. Others will resist common sense measures like armed security because it bursts their world view wide open. You also have the people that refuse to protect others, they leave keys in locks, prop open security doors so they can smoke, that sort of thing. The hardest part of any security plan is protecting the people that refuse to be protected.