Metal detectors and security checkpoints at all schools.
I definitely agree with everything else you have said but honestly, I'm not sure how well this would work. Why wouldn't the shooter just start shooting at the checkpoint? Perhaps it would cut down on the number of deaths, but it feels like a band-aid rather than a solution that will change everything. But then, as you said, it seems policy-makers are unwilling to take any serious steps, so maybe beefing up defenses is the best thing we can do.
