As a teacher, I've been doing lockdown drills for 15 years now. All of the procedures are designed to buy time until police arrive.
Now all of those procedures have to be reevaluated. I've thought through countless scenarios with our building layout and my classroom and what options there are if something goes down. Now I have to add on the "well what do we do if the cops ignore us" to the equation.
I can't count on the police. That's the reality now. If things go down, we are on our own.
And before someone inevitably comes along and says "well statistically you never have to worry about it, blah blah," I can't afford to not think about it or be prepared. In 15 years teaching I've had 3 students I thought capable of a school shooting. 1 of them I'm convinced would have done it if he ever got hold of a weapon. That one eventually grew up, got denied from joining the marines, and ended up killing his grandma and going to prison.