Michael Schoch, Answers questions on Noodle
That sounds like a tough situation for both you and your child. The best and only real solution is to aim for effective communication.
I recommend reading this article on dealing with difficult teachers as it touches on some of the messy realities behind parent-teacher and teacher-student relationships.
As the article mentions, staying calm and considering your word choice can go a long way towards resolving the issue. It's important to have an accurate description of what's going on in the class. Try to make sure that the conversations your child is recounting for you are free (or free-ish) from exaggerations and misrepresentation. If you choose to speak with the teacher, try to stay as calm and receptive to her/his side of the story as possible.
In the best case scenario, this may have been a misunderstanding that can be resolved through a quick meeting or phone call, either between your child and the teacher, or if it's necessary, between you and the teacher.
If the teacher is adamant about not sharing the rubric, despite your best attempts to speak calmly, then it may be time to contact an administrator.
There are several other articles on the Working with Middle School Teachers page that offer advice on how to handle tough issues like grading and class policy when speaking with a teacher. Hopefully the page gives you some ideas for how you and your child can best approach this potentially tough or uncomfortable meeting.
Best of luck and feel free to write back with a clarifying question at any time!