Like I said, if political zealots won’t accept any other cause than guns then that’s where we are. We can’t solve this problem. So harden the schools. Metal detectors, magnetic door locks. Security ID badges that open the doors. Cameras. Maybe even a security station that monitors everything.Lots of times I've stood in the fish bowl of a secure factory waiting for somebody to get to the front desk and let me in. After a while I would call my contact inside and be directed to the "block of wood" door. I think it's something that will always be with us unless like jamil says you issue fobs or cards. Even then you would need a procedure to control those.
Absolute control would need a security center that would monitor all the doors, like a prison. Maybe that's where we are headed with schools.