An elementary school in Israel. Prepared to improve the odds. (Thanks)
Have you ever noticed that all the crazies who want to go out in a blaze of bullets before the final round that ends their life, never, ever, choose to go to a police station and try to shoot all the cops?
Realistic response to a threat = teacher has a carbine. Idealistic response to a threat = lock the doors and call "first responders". Silly response to a threat = gun free zones.