Deputized Security Officers
My idea is a deputized security person: a plainclothes officer on site watching monitors of each entrance at a school, can spot intruder, notify police, hopefully take out the shooter (head shot, they wear body armor), or occupy them until backup.
A sonic gunshot detector is easy, wireless alarm to police IF a shot occurs in a school so they know right away, that should be minimum.
If multiple shooters, say four at four different doors), the DSF officer makes a decision to coordinate from within the security room or do what he can solo because with the advantage that he knows where each is, he can at least slow them down, plus he's in wireless contact with police throughout.
Parents (through the PTA) can take donations for the initial security system, not that much - you can likely find retired volunteers like myself that won't need a salary.. maybe two four-hour a day volunteers, it may be stressful and you don't want to leave. A microwave and fridge should be in the room, and hopefully close to a restroom.. Let's face it, if thousands of small retailers can have these security systems to save their merchandise then WE can do this to SAVE KIDS - A NO BRAINER - WE CANNOT SAY THIS IS TOO EXPENSIVE, NO EXPENSE IN THIS AREA IS TOO MUCH. The kids will know they have a helpful watcher ON SITE, hopefully in a hidden location not easily cased by someone planning an attack.. eventually there'll be a coordinated terrorist attack and this will help with that as well.
In Peter Berg’s film KINGDOM OF HEAVEN, after the first responders showed up to an attack by shooters in an SUV on a softball game, the terrorists had a bomb ready as an arriving ambulance, and set off a secondary explosion that killed another few hundred emergency personnel, including the FBI officer in charge, the first attack was just a draw for that.. everyone needs to keep that in mind as well.
Of course, these will be trained personnel with no history of any dangerous mental conditions (hey, we've all had the blues) just like you would for any security officer that may require arming.. you can likely get lots of ex law enforcement and ex-military as volunteers.. fight fire with fire, this are assaults, we can't wait until after the fact to send 500 armed police in, at that point its just another waiting target and lots of overtime pay.
Someone mentioned the idea of arming teachers who qualify is also a good augmentation to the Deputized Security Officer, but of course they will not have monitors or body armor so the teacher will possibly be another victim, less likely for the DSO, but any additional firepower will help and slow down the attackers.
Also getting a wireless radio system on each custodian is a good idea, so they can also communicate with law enforcement - this technology is cheap nowdays and we are the technology kings so we can do it.
If I was a DSO, I'd want both a pistol and a sniper rifle w a scope, it may take a head shot from a distance to take out a shooter, but the pistol may be your only option depending on the situation and how fast you have to respond.. if possible the security room can double as a sealed panic room if say it's a coordinated assault w multiple shooters and the DSO can best help by coordinating police from within from a secure location with multiple camera eyes.
So far, no group of terrorists have done this, but YOU KNOW THAT'S COMING - we have to prep for that NOW, don't wait until after the fact because "the feds cut our state budget" --- GET PEOPLE TO PAY - IT'S OUR KIDS, WE CAN PAY FOR THIS, screw the budgets.