I was idly wondering if gangster movies (Godfather, Goodfellas, Sopranos) made it "cool" for impressionable young men to shoot up a room of innocent strangers.
Which led me to idly wonder if there wasn't some way to discourage that behavior. Obviously arming all kindergarten teachers is a brain dead suggestion.
Then it struck me: sprinklers. Aren't almost all public spaces provisioned with fire suppression sprinkler systems? All you have to do is set off the sprinklers at the first sign of trouble. Non-harmful to the bystanders, discouraging to evil-doers, property damage only.