Insurers will shut down any proposal for teachers to arm themselves. No insurer is going to want to insure an armed school. The teachers are far more likely to be shooting each other, kids and administrators than to deter a mass shooting. They will have to pay a premium for the additional risk, and when push comes to shove, the school board is not going to allocate resources just to have a "wild westy" school.
To answer an earlier question, how many teachers secretly bring a weapon to school, the answer is none. Its because they don't want to get fired for being caught doing so, not to mention being in the front page of all the newspapers.