I can't remember which situation made my school do this (either because of the shooting in Virginia or an actual threat), but near the end of the year last term the VP locked all the entrance doors except the front doors, of course, at 9 every day. It started with the side doors always being locked. Which made coming in at 7:45 every day a little more annoying.

Another situation: there was this girl, she goes somewhere else now, who would tell me "If I ever tell you to leave the school, leave." every now and then. I brushed it off, I knew she wouldn't do such a thing and she hasn't. If she did, I would be able to report her immediately and the cop station isn't even 5 minutes away from the school