Generally its for both the school and the police. And if you want to talk about cost benefits, how long would it take to convert that warehouse into anything that would remotely resemble a school? And at what cost? Because an empty warehouse is just that, empty. No rooms, no furniture, no hallways, no fatal funnels, no blind spots, no doorways...I could go on.
And you tend to want to practice where the real deal might happen.
Again, I agree an announcement could or should have been made at the start. But the comments from the parents reek of anti gun politics. "OMG! The police had a dastardly AR15! OMG! OMG! Think of the children!"