Your sitting in class and some random dude just strolls in and shoots the teacher in the head. Then he points two guns at you. Is there really anything you can do at that point?
Some people had an opportunity to act and they certainly did. Some guys blocked the door with a table and lived to tell the story. Others never had a chance to react. As for me, going on the offensive against a crazed man with two semi-automatics unloading 200 rounds of ammunition is not a choice I would ever make. I agree that there's no shame in running; that's probably what I would've done.