You make good points, it would be difficult and expensive. An armed single entry gate was what I had in mind. But it wouldn't be completely useless or even harm

You make good points, it would be difficult and expensive. An armed single entry gate was what I had in mind. But it wouldn't be completely useless or even harmful, like arming teachers would be. It's also true that the shooter could just go the mall or a cinema and get a similar result. Attacking the school does seem to have a particular significance however.