Which reinforces the fact that armed resistance, ANY armed resistance usually causes the shooter to take his own life. So the schools need parents, teachers and admins with concealed firearms.
From what I can tell there are basically two types of active shooter.
You have the shooter that has a target. There is someone or group who they have a problem with. They will ignore anything that isn't their target or getting in the way of their target. Once they have achieved their goal they typically kill themselves. The guy at LAX who had a problem with the TSA would be an example. He ignored hundreds of other easy targets and only went after the group he was interested in or those who tried to stop him. In this case, your best option is to run away. If you aren't his target, he won't care. If you are his target, you want to be somewhere else anyway.
Then you have the shooter who wants a high body count. It doesn't matter who is in front of them, they will shoot at it. This group seems to kill themselves as soon as there is any resistance or reality starts to diverge from their mental script. In this case, you want to run away since you are a target anyway.
Sheltering in place would work great in the case of the targeted shooter, but is the last thing you want to do for the kill-em-all shooter. It looks like the best bet in all cases is the run-for-the-hills approach.
Keep in mind we are dealing with mentally unstable lone wolf types here. They most likely have little practical experience with their weapons or skill in the type assault they are carrying out. There is no sniper in the woods or second man to pick off those who are escaping. Even if there were, your odds are still better moving than standing still.
Even with all this, the odds of being involved in an active shooter scenario are astoundingly small. Your odds of being a casualty in an active shooter scenario are even smaller. Your odds of being killed in an active shooter scenario are smaller yet.
Don't live in fear. Move away from the noises. Fight if cornered.