Once a school requires/allows teachers to have guns, the chances it can find the insurance schools are required to have get much lower. And if can find insurance, its rates will go up up up up up. Because, just think of the lawsuits that will be filed once a poorly-trained teacher kills a child while having a shootout with some suicidal gun lover. That's some kinda potential liability!
And the money to cover those insurance premiums will come from taxpayers - the same taxpayers who will refuse to allow their taxes to go up because of Freedumb.
Want to bankrupt public schools? Arm them.