I am more familiar with colleges and universities where there are appeals mechanisms for this type of misconduct by a department chair or dean. I would encourage you to bring your complaint to the school board and/or your union. This type of conduct by a principal undermines the credibility of grades issued by that school, which can have consequences for students applying to colleges and universities. (Does this mean that you have a substitute who will complete finals and course marks? If you are blocked access, how can you properly communicate with your substitute?) Let me know how this works out.
