We had one of these in our school after Columbine... except we were told that it would happen and that it was just a drill, just like all the fire drills we had. Personally, I think it's a bad idea to have drills for things like this: In the case of a fire, there are clear rules that you can follow to survive, and going through the fire drill routine thinking there's no fire, even when there is one, works just as well as when you know there's a fire. All this excercise will do is decondition the kids to the danger of an actual attack by a gunman; There's no clear-cut, rote way to safety in an attack by a crazed gunman.