You can't do anything about it . We have what is called a "closed shop"!! If you don't pay the full dues because you don't want to belong to the union you still have to pay 85% because they do all the barganing for you . On top of that you become a sitting target for a toilet fish principal or supervisor if you don't have union representation.
Why has it not become obvious to you that you need to teach somewhere else or to find another line of work entirely ?