If I were a teacher, and could legally carry, I would. BUT, I wouldn't reasonably expect a teacher to carry a handgun and defend other people with it, for many reasons - interest, mindset, training, liability. I woudn't volunteer to protect others, outside of my family. I may protect others, but I don't want the obligation.
I think it's much more reasonable to establish a security office within each school district and/or county, staffed with sworn, armed security guards (possibly attached to the local police or sheriff's office), trained just like the LEO's, equipped just like the LEO's, with the same legal protections. Assign one or more guards to each school as required. Totally locally controlled, no federal government, no state government involvement. Funded by the taxpayers (who pay for everything anyway).