What about the reverse situation, if a church is used as a polling place, then the pastor of that church shouldn’t serve as an election judge? You are applying a standard based on the location of the meeting that violates this principal’s right to freedom of religion.
Well, the issue with the Principle is that his actions violated the policy of his employer, the school board. I guess the same would apply to your question. Do some churches have policies against their Pastor's, or other senior administrators, doing party affiliated political work? I don't belong to a church, so I don't know the answer.