Several times since Trump's earlier statements, I have made clear my position that teachers and school officials are unlikely to be armed. However, in my most recent post on the topic, I argued that the mere fact of Trump advocating the policy put it in "the Overton window," and the political system then responds by treating it as a more viable policy than it is.
We are now seeing the effects of that. Betsy DeVos is... not someone we would normally take seriously. She was very nearly denied a cabinet position because even her own party came close to recognizing that she isn't qualified. She has announced plans to assist schools that want to arm teachers and officials. Remember that not only do schools not want to let teachers carry guns, they don't want to let Betsy DeVos in their doors!
This is a shiny thing. This is a distraction. You can't force a teacher or administrator to carry a gun when they don't want to do so, and the proportion of teachers and school officials disposed to carry firearms is vanishingly small. Telling teachers they are allowed to carry guns would be like telling members of the Trump family that they are allowed to talk to Robert Mueller. See my point? Put this at the school-level, and the idea gets quashed, except in places where they hand out Darwin Awards like Boy Scout merit badges.
Notice how effectively this has shifted the debate, though. Betsy DeVos continues pushing the arm-the-teachers idea, and that defines the gun policy debate through context.
Just because it won't happen doesn't mean it is irrelevant.