Then that might be a little low. With pay, benefits, training, etc. Amazon warehouse workers cost about $75k, and that seems to be the new standard that everyone looks at. You can go below that, but you have to offer better conditions.
So what are the conditions? I can imagine a school security guard that literally does nothing. He stares out in space most of the time, and so would cost a lot less. It would probably be best for everyone if he was not armed.
But if you intend him to actually engage a school shooter, investigate possible crimes in the school, deescalate confrontation with emotionally troubled students, etc., you probably have to double that, so $150k. Firearms training alone needs to be constant(at least once a month). The training necessary not to hit an insulting teenager might be even greater. Investigating possible crimes also requires a lot of training.
You might say we could use people on disability from the military or the police. That way we get the initial training, and benefits for free. They have fewer other options, so would be willing to work for less. The problem with that is dealing with obnoxious students is very bad for PTSD. You are creating an obvious violence problem.