I certainly see what you're saying, but if a shooter enters a school, there is virtually nothing a small child can do to help keep himself safe. Small children DO seem incredibly compliant when danger occurs - that photo of that class of children all holding on to each other and running toward the fire station just makes me cry - they were SO WILLING to follow instructions totally. ALL of them are running, ALL of them are holding on to each other with both hands. Children in danger do seem to follow leadership without question.
I saw someone else in this thread grew up in the cold war, as did I, on an air force base. We had monthly drills where we had to "duck and cover" and other raids (with a different siren sound) where we all had to exit the building quickly without trampling each other. A fifth grade teacher was the first to bring it up to the administration that kids who went to that school were thoroughly traumatized after years of this. Even as a child I knew that hiding under a desk if the school were bombed wouldn't save me. Just the monthly reminder that we could be bombed - was awful and traumatizing.
I do see what you're saying that if kids have heard about this they need to know what to do - but in fact, there's nothing at all they CAN do it if it happens. I just think telling them this is rare, won't happen to them, the shooter is now dead, is best.