Yeah, if you are unfortunate enough to be in the first place the nutso psycho starts blasting people, it really won't matter if you have a holstered gun or if it is in the bottom of your gym bag. Once they have the drop on you, it is usually over before you can respond. I think one of Henry Fonda's characters was quoated as saying "don't ever draw on a man with his finger on the trigger". I would probably still try anyways, even if he is already blasting away, but the chances are not much better than if I had to go fishing for it at the bottom of a bag. Same goes for an orgainzed rush of the assailant. Unless you are in a group that is actually anticipating such an event and has planned for it ahead of time, there's really no hope of executing such an organized counter attack if the shooter has the element of surprise.

The bottom line is if you have the time there's lots you can do to improve your chances, and the best would be to retrieve your own firearm in time to bring it to bear against the unsuspecting nutso psycho. Those who successfully barricaded themselves into their classroom preventing the nutso from entering only mitigated the threat to themselves, and didn't do much to stop him from victimizing others less fortunate down the hall.

Even without a firearm, you could try and Macgyver something up that would have a chance of neutralizing or inhibiting the nutso from further aggression.