Not to take anything away from the behavior of the teacher, but the apparently unflinching submission of the students bothers me.
Much like the lack of opposition by the Jews during the Nazi round-ups, which were explained as "What good would opposing do? We were going to die no matter what we did," I can find nothing that suggests that any student(s) made any attempt to escape from or attack the teacher once the gun was presented.
"Never again!" makes a great slogan, but it requires action both on the political and the personal level. There are documented reports of at least one student and one professor at Va Tech who rose up against Cho and his plan to shoot everyone as they cowered under their desks waiting for him to walk up and kill them. There are also documented reports of several students who climbed out windows to escape.
Having been shot at I know how difficult it is to do anything other than stand there flatfooted in a combination of fear and confusion. The desire not to die served as a motivation to "do something!" whether it was to drop to the ground, move "off the X," or to return fire. For the record, one does not need to have a firearm to return fire.
I know not what course others may take, but as for me ....
stay safe.