zxcvbob is right. I know of many teachers at my school (I'm a HS teacher) who have military background, CWPs or are avid hunters. Lots of us have experience with weapons. Just because we love teaching doesn't mean that we aren't serious, deliberate people who are capable of decisive action.
We don't need to be Delta Force. We only need to have an effective means of defending our students at our disposal. If something like this were to happen in my school, I would like to know that someone has a fighting chance of stopping the BG.
As it is, I train my classes that we will lock ourselves in a storage room if we hear a lockdown called. If an active shooter enters our room before we have a chance to hide, we will throw everything that isn't nailed down and rush him en masse. Some of us will probably die but we won't all die and that is the point.
To a much greater extent than most mechanical devices, firearms are terribly unforgiving of any overconfidence, complacency or negligence.