Volunteer vets would be cost effective but the turnover could lead to confusion in a crisis...teachers and students not recognizing the volunteers, volunteers not being familiar with the buildings layout, not knowing where students are...Ect...I think you would be able to find retired LEOs or veterans who would do some volunteer work a week at time. Rotating volunteers to watch over the schools.
The students shouldn't know who carries and who doesn't.
I know many career educators and they're not all lemmings. I've shot with members here who are teachers and trust them implicitly.
Far better in a crisis to have armed immediate responders that the kids know and recognize as there beloved teachers...particularly in a high school where male students are more that capable of taking a full grown man off his feet "if they mistook him for a threat".
Our schools are like large extended families...far better for scared kids to get direction from trusted and armed adults rather than strangers.