At Columbine, the security officers were in uniforms, easy to pick out, easy to deceive and lock out of the school by the perps. Concealed carry is the work. Off duty officers in plain clothes, volunteer parents who pass a training test in plain clothes (like someone coming in and out of school). Teachers with concealed carry, that have been "properly trained". All of these can and should be used. Voluntary military personnel from a local military base during their off duty hours. It would not cost a fortune, just some type of certification background (for the left to be satisfied). Arguement should be made that they spent the money for air marshals and bullet proof doors to the cockpits of planes, scans and screenings of those entering.
Another would be hardening the doors, windows at schools, like at a bank. Piano wire mesh in windows that look better than bars, or lots of small narrow windows that someone can't squeeze through. Bullet proof doors and locks. Security cameras everywhere with monitoring in the office with a volunteer security guard watching all this.
Lots of things can be done. Volunteer armed parents, guards, etc, is the cheapest until some of the window and door improvements are made.
Home schooling with a local school monitoring the progress say weekly should also be encouraged. With computers, homework assignments etc, being done and turned in to the school for checking. Say the kids have to go in once or twice a week for testing and checking of their learning progress. Fewer kids at school at any one time.
Also encouragement of private schools meeting mimimum public standards in testing. Private schools could have armed teachers and parents with no liberal backlash. Most are conservative anyway. Vouchers, and tax credits for these kids. I know the AEA doesn't agree with this, they loose their "power".