My wife and I attended a town active shooter discussion a couple of weeks ago. Several officers from surrounding towns were present. I wasn't sure what to expect, maybe a lot of chest beating. The officers were pretty humble and said that on any given day there are not a lot of officers on duty considering the geography they need to cover. Average response time is about three minutes. He said three minutes feels like hours when someone is trying to kill you.
His main point was that you need to assume help isn't coming and take care of yourself. They did say that denying entrance is the best defense if you are able. If he gets in then throw everything that you can get your hands on at him to distract. At that point whether to fight or flee is a personal choice.