I think it would be better if there could be more organisation during an actual shooting event, classes too close to the shooter could simply lock the door and stay put, whilst other classes which are far enough away could make a safe exit quickly. This would rely on some knowledge of the shooters position and direction he's heading and some ability to communicate this information to all the teachers, maybe read out the shooters movements over the pa system, could annoy the shooter though...

It would probably be a really good idea to have quick exits for all classrooms possible, fire exits on ground levels and stairs built to classes off ground level, its a bit of expense but probably worth it...