It's an elephant, eat it one bite at a time. Start small, focusing on small gains. Allow willing staff to CCW, and give them a simply constructed, credible training product that allows them to defend their personal and classroom space against bell curve problems. As you establish safety and efficacy, take the next most palatable step in your venue. In the overwhelming majority of districts, if you walk in talking about hostage shots, rifles, QRF concepts, and the like you were done before you began. Please don't do that, you're making it hard for the rest of us.
A teacher that is taught to shoot a 5^5 competently and position themselves a few feet from their classroom door can be a formidable defense of ~30 kids. Start there.
I don't know who to credit, but another poster/blogger discussed this as a "zone defense" approach. I like it, and am changing some minds with it.