When I was a teacher/librarian ('70s/'80s) in Tracy, California, we were concerned about shooters coming on campus.
I told the administration that security guards were a waste of time and money. The only thing that would be effective is installing security cameras to spot the shooter before he enters the campus. I went to BART (Bay Area Rapid Transit) to view their security set up and made a presentation to the administration who felt it was too expensive.
If schools would install this system, one person could secure the school and spot a shooter before he enters and push an alarm for a full campus lock up, notify police, and if he is properly trained he can then leave with his AK-47 and protect the school.
Today the cost of security cameras is a fraction of what they would have cost at the time of my proposal. Please pass this idea on to anyone who can make it a reality.