Cuz they could hear the shooting and screaming down the hall, plus, he wouldn't just be standing there with the kids, he'd be shooting them.
You will quickly find that you are attempting to reason with someone who has an almost religious faith in the system, procedures and rules. If you look for it, you can find at least 3 on this thread.
To say that the teacher should have done the =decent= thing and opened the door is Heresy. There is no logic beyond that.