I'm not saying it's the best way but, if a local government insists on arming teachers, I think they shouldn't do it haphazardly:
1. Choose a body, such as the state police, to design and run the program.
2. A small group of volunteers from teachers/staff are vetted and receive special training.
3. The state police are responsible for maintaining the guns, which are secured in certain locations of the school, along with a bright ID vest which identifies them to students and responding police.
4. At any time, some of the volunteers are "on call" and have keys.