I understand the point you're making, and for myself, I agree - b every time. However, as a teacher, I don't feel that it's a decision I can autocratically make for every student in my classroom. Too much depends on the age and abilities of the students, the willingness of the teacher to lead, and the situation occurring at the moment. Are too many people trained from an early age to lay down and hope for the best? Yes. But that doesn't mean it's always a bad response. You have to be willing to shift your plans according to the situation as it is occurring at the time.
As the teacher you would be in a no win situation, and would have to live with yourself, if you survived, with the consequences of whatever decision you made. In a this kind of situation, I would have to do something, realizing that I am acting alone, and would also have live with my decisions or die by them. The others in the situation would have choices to make as well, I would hope they would choose to take advantage of whatever distraction I can provide and get the hell out, but I suspect there would be those that would follow me and those that would freeze.
I speak only for myself, but I cannot assume that a person coming into a school, restraunt, place of business, public conveyance or anywhere else, with a weapon drawn, is not going to use that weapon simply because people are not moving. My number one choice is escape, always, but that isn't always an option, and waiting for someone else to do it has never been my forte.