Given the millions of concealed weapons permit holders and the thousands of crimes that they have stopped with those concealed weapons, this has yet to happen.
It's a stupid scenario anyways if you think about it, let say I am in class sitting next to Bob whom I have been sitting next to for the past two months and other than some strange grooming habits, every thing indicates he is a normal guy. We hear shots ring out in the hall or worse the door crashes open and shots are fired. I hit the floor reach into my pack back or smartcarry holster whatever and pull my .357 snubbie, I look over at Bob and he produces a 1911, I immediately think:
a.) Shoot the beady eyed bastard I always knew he wanted to kill us all.
b.) Think, "thank frigging God he's armed too I might survive this".
c.) I wish I was carrying my XD45 instead of the stupid 5 shot snub.
d.) Come up off the ground gun at eye level and pushed out, focused on the front sight, pulling the trigger once the gun man is made, remembering all of the while, slow but steady wins the game.
The anti-s would do A. I would like to think I would do B, C, & D.
Another point the nay sayers like to make is my God the police will shoot you instead of the gun man, this has also not happened but it is a good thing to think about. Once you have stopped the threat, holster your weapon out of sight and resume your status as scared student, IMMEDIATELY. Most of the time the cops will be there long after it's all over.