Other than school administrators and staff seeming to be complete idiots when it comes to running things, I assume it has something to do with the majority of people tending to do stupid shit when they panic or are faced with a potentially bad situation. In my sister's senior year of high school, they got a call about a man wearing a trench coat with a rifle on the school roof. So they evacuated everybody to the tennis court (They may have evacuated the junior high and/or elementary school there too, I'm not sure. I honestly wouldn't put it past them though.), where he'd have an almost completely clear shot at everybody. I'm pretty sure the tiny trees they have between the parking lot and the court wouldn't have done a thing for protection. Premier school district of PA my ass.