I've seen this too many times. At my mom's assisted living someone would prop the door at the end of her corridor open with a stone so they could go out and smoke and get back in. Its convenience. I understand that. But that's all it takes is a moment of "convenience" to let a shooter in. Especially one that knows ahead of time that's what the staff does. I reported it. Your school should have made an incident report. Is Joe going to feel bad when kids get killed because he left the door unlocked? Yes! He will feel terrible. He needs a reminder and maybe a sign on the door of warning.