Better yet provide a free lunch, breakfast, afternoon snacks/beverage to police officers, sheriff's deputies, State Troopers etc. at the school who can come and go as they need to.
A camera system that can be monitored by the police in the event of a problem.
Always have a police car parked near the lobby as a deterrent.
Use police communications and training to shut down a shooter immediately.
A Special alarm that differs from the Fire alarm. Kids don't respond with any real urgency to 'just another fire alarm'.
Bullet Proof glass/plexi and bullet proof doors on every school class room. Turn every classroom into a place of safety in seconds.
Summertime exercises by responding officers to get thoroughly familiar with the schools they are responsible for. Make sure a select group of school administrators/ coaches/teachers participate with the officers so that they can understand how to keep students safe.