Some years ago a distraught mother of a student brought a gun to school and actually shot one of the faculty - in the leg or hip as I recall. She was tackled by one of our administrators!
Not too long after 9/11, I walked out into the wide hallway/lobby entrance in our building and there was a bookbag in the middle of the hall. No one was around, no one seemed to belong to it, so I called security. I checked my faculty handbook to confirm that teachers are not allowed to do things to get themselves blown up. By the time they got there, a woman came out of the women's restroom, picked up the bookbag and started walking off. Anybody could have walked off with her unattended bookbag. It was right in the middle of the hallway. I had a little polite, respectful, talk with her; she was embarassed.
There was a similar incident where a bookbag was left in the same hallway but near some couches. I watched and waited, called security and finally a guy comes and picks up the boodbag, security and I both had a talk with him.
Then, not so long after Columbine, I'm going down the hall and see a guy approaching with a long black leather coat on. It was too warm for the coat and his arms were not in the sleeves but rather concealed inside the body of the coat. I did not like that. I watched where he went and then went on to my class. I wondered through the whole class period if we were going to start hearing booms. We didn't so I dismissed it. Two days later, here comes this same guy, in the same coat with his right arm inside the coat exactly the way you would conceal a long gun under this long black coat. The next day, I couldn't stand it any longer so I reported it to security. The next class period they were literally out in force but discrete. This time the guy doesn't have the coat on. If he had, I'm quite sure security would have had a talk with him.