Purchase security where? .........At school entrances? That would require hundreds of thousands of security officers.
Let's say you could find enough people to do the job. Dude at the entrance might just be the first to get shot. Seems to me, to truly make schools "gun free zones", they'd have to build 'em kind of like prisons. Metal detectors, cameras, walls, security guards. And theoretically this would only stop shootings....... inside school. They could still get shot getting off the school bus or waiting in line to get inside.