A lot of proposals made, most of them not very realistic or socially or economically acceptable, other than emotionally. My wife and I were talking about this last night and concluded that one single small action that would "harden the target" signficantly would be having classroom doors that automatically lock such that they can be opened from the inside, but could by opened from the "outside" or hallway side with a key. That's not very expensive, and it doesn't save everyone's life, but it would keep a classroom full of kids from being targets. So it could save a significant number of lives if and when an intruder comes into a school to shoot.