Supposing, though, you did manage to persuade teachers (and school administrators and school lawyers) to carry guns: there's lots of details there to take care of. Do the teachers carry them on their persons, in holsters? In their purses? In a locked cabinet in the classroom? What happens
Probably a better solution is what the airlines came up with in the wake of the 9/11 terrorist hijackings: making access to the cockpit impossible by "hardening" the access door. The same could be done for classrooms: metal doors with no windows, with a heavy metal bar on the inside to keep a gunman out and the kids/teacher safe within. Periodic drills to familiarize the staff and students with the necessity of getting inside the secure classrooms.
Limit access into and out of the school building itself with more hardened steel doors. If the school has a "resource officer" (read: cop), he should be stationed at the main entrance so as to monitor people arriving at the school.
Well, those are my ideas. Feel free to contribute others as you like.