If properly trained volunteers, yes. And I mean trained in firearms use and self defense law to the level of a peace officer.
They have an emotional connection with their students, thus more incentive to protect them.
As well, they will be in the school, and a target, therefore they would be defending themselves.
The type of person that becomes a teacher is most unlikely to want to kill people. You can bank on that. The whole idea of arming teachers is bizarre, to say the least.