If armed faculty means safer schools, why can't they get insurance coverage?

This actually has very little to do with the gun debate.

Schools have liability insurance. If an accident happens with an employee's gun, the school will be sued and the insurer will have to pay out. If a school is shot up by someone else, the school isn't liable for anything--it's hardly their fault someone else committed a crime. So the increase in risk of accidents hurts the insurer, but any safety benefit doesn't help the insurer.

In short, an insurer has a financial reason to be against it regardless of the effect on safety.