Yea, I have to say, one of the things that I always find about school shootings is that the gunman just goes in and sprays as many bullets as he can, and if someone happens to get hit critically, they go down.
This guy, on the other hand, was stone cold - he judged his shots and actually aimed to deliberately kill. If you read the story, he walked into the room and shot the teacher in the head with his first shot - obviously he didn't go running in and shooting wildly - he knew what he was doing.
Now, granted, if I was lined up against the wall I'd rather charge him and get shot doing that, then waiting for him to walk over and pop me in the head - but I know that as soon as I broke from the ranks I'd be shot down instantly. Sandwich, a quick image search of "Virginia tech classrooms" reveals:

and

An environment like that is basically a shooting gallery. Either you go down and hide under the desks and pray that this guy thinks you're already dead, or you stand up and try to charge him. If you do the latter, you're now the biggest target in the room, and get priority over everyone else - because he knows if you're standing up you're probably planning to attack him.
While I would have charged and gone down while doing that (at least, that's what I think I'd do), it's very logical to be "sheep-like" as you so put it, and stay down and hide under the bodies. Even then you wouldn't stand much of a chance, because as we can see, this gunman was determined to take out everyone in the room, even if he had to come back multiple times to do it. It's like he studied other shootings and found out what they did wrong, and improve on it.