Probably The Cheapest And Least Silly Way To Defend Schools From The Almost-Zero-Probability Threat Of A School Shooter

Nevada proposed a similar plan before the economic downturn of 2008-09:

North Carolina lawmakers are once again pushing to arm teachers with guns, and they're trying to make it worth teachers' while.

Filed on Wednesday, the School Security Act of 2019 would give a 5% salary boost to teachers who undergo basic police training. 

These "teacher resource officers" could carry guns in an open or concealed manner, and they would have the same arrest powers as police officers, the bill says.

Nevada's plan was to make these trained teachers "reserve sheriff officers", with no law enforcement powers until they were brought onto active duty during an incident.  They would be deputies on the scene until full-time law enforcement showed up.

Personally, I prefer the Nevada plan.