December of last year, we had one at my high school. To answer the question of how chaotic it was...

I was in the weight room, goofing around when we heard the first shot. kids from the basketball court started pouring in, it took me until the third shot to realize what was most likely happening. Our two teachers, both pretty burly men, slammed the doors shut and ushered students between the two entrances along the wall. Time flooded together, I was doing my best job at comforting girls who were breaking down, in part because we needed them to be quiet and in part because that was easier for me than being scared myself. People began shaking the door, we assumed it was gunmen, they were trying to get in. (Later found out it was our campus security checking the door). A few more gunshots, screaming echoing through the hallway, a few guys had their fight instinct triggered and were standing alongside the teachers, positioned to attack anyone who came in either door. The real chaos was looking at guys who were big, lifted a lot and played varsity sports, sitting with their heads between their knees. Seeing real fear in people's eyes is what scares you. We were on lockdown for just over 3 hours, it went silent for 2 and a half. No one spoke. We just waited. Finally, we heard banging on the door, a loud bang, and a swat team flooded in with guns pointed at us, yelling at us to show our hands. Those of us standing near the doors put our hands up a-real quick! Two seconds of silence, then people busted out in tears. They let 3 hours of emotion break free. We were evacuated, escorted to a church near by where we were to wait for our parents. Even if we were 18 we were not allowed to leave. They pulled together a make-shift system to try and establish some organization, but it wasn't very successful. Then you just wondered the room, trying to find people. We didn't have much news on deaths, wounded or anything of that sort, so everyone was just looking for their best friends or boyfriends amid the chaos.