Maybe I would not try to pass any laws which prevent nothing but punish law abiding citizens. I would get rid of backpacks because carrying heavy backpacks is not good for any kid and it is not required anyway. Kids will bring nothing with them. Everything will be in the school. School will have a card scanner system combined with a fingerprint or retina scanner. Yes, this will be expensive but we have to start investing in the security anyway. Metal detectors at the main entrance which will be supervised by staff, not the outside company. Staff knows kids.
The advantage in schools is ability of children and the staff to recognize intruders. That is a huge benefit as oppose to airplanes, for example. If there will be a problematic child in the class this child will have a very hard time to bring anything metal in the school. Yes, he/she can decide to choke the other kids, but that will not result in high casualties.