The time has come to arm every teacher willing to carry. Important is proper training and practice. Yes, routine practice so that they shoot to kill an active attacker effectively.
That means regular time on the firing range, just like police officers. And yes, I would pay for that. If hostile antagonists realize they will run into opposition at a school, I believe that will be sufficient deterrence to stop 90% of school shootings into the future.
If I were a teacher, I would want the ability to carry to protect myself and the students entrusted to my care.
If the teachers keep it to themselves an attacker would be even more reluctant since he wouldn't know where a potential deadly response could come from.