I've heard the alternative explanation that's a lot simpler: after Columbine a lot of schools did a bunch of bullshit drills and security theater stuff. But one of the things that mattered was just ensuring all classroom doors had keyholes on both sides. That way a teacher could lock a door. It was a cheap (I'm guessing like $10k ~ $20k per school .. feeds the local locksmith business) and made it seem like you were doing something.

Sandyhook didn't have any of that. Students were found crammed into corners because teachers couldn't lock the doors. Sandyhook was demolished to cover up plain ole negligence.

That's the Robert Barnes explanation anyway. I honestly haven't looked that into it either way. Seems plausible.