How can I receive fair representation from a union that does not practice impartiality?

Looking for EMPLOYMENT or LABOR Law help.
I work in a school district where the majority of employees are related to each other. This has left me without fair union representation for a meeting that my Principal has "directed" me to attend. I feel that my only other option is to hire a lawyer which I doubt I can afford.
What can I do to make sure I have a fair and unbiased representative in the meeting with me? Do I ask my Principal to suggest a viable alternative? I can only continue to reschedule this meeting for so long.
You should also note that this meeting has been called because I brought to my Principal's attention that another employee has never, ever handed in her substitute form (whistleblowing? The person I turned in to the Principal's attention was his secretary's cousin) and I felt that it was unfair that he was demanding mine immediately without addressing the bigger problem.
It has turned into a childish power play where the Principal runs around the school interupting the class that I am in to get me to sign letters stating that I postponed a meeting due to lack of representation. On his part, I feel it is a case of lack of management skills which is beginning to feel a lot like harassment. As for me, I do not want to attend his meeting alone. I need fair representation. Where/who do I turn to given my present circumstances?
How do I proceed?
Thank you for any and all information/advice you can give me.