I gave some thought to this since the news coverage began. I also considered that if the six girls in Colorado had rushed the guy it would have been the best thing. Pick up a chair, go for the privates, figure to go down fighting, whatever. I speak from some experience, and know that big guys have sometimes backed down and run. (Not to mention what the boys might have done before being told to leave).
It's like the flights on 9/11. Backing down, following demands, and waiting only works when the person holding you at gunpoint doesn't want to kill you. If they do want to kill you, then you are simply helping them do so on their terms and you've missed the initiative to take them down, which is when they first start waving their weapon around or attack someone. And since the Montreal shooting years ago, it's been clear that any time a male gunman asks all the boys to leave a classroom, things are only going to go downhill if people surrender and comply.