Several have voiced similar thoughts and I'm inclined to think this is about the best the kids could be expected to do. The suggestions to turn off cell phones are good. IMO picking up an improvised weapon on their way to cover shouldn't be completely ruled out if we are talking about kids that are in their very late teens.

I did read about the shooting of the off-duty police officer but what I read made no mention of whether or not he was in uniform or armed. I know a single armed responder may offer little protection in some scenarios but they have a much better chance than an unarmed officer. Around here any gathering of that many kids would almost certainly have at least one armed officer on duty. I hate to say it but this would be particularly true if the kids were from politically well connected, upper middle-class or better families.