How do you deal with a tough bossy 11 year old girl?

I worked with the higher special needs kids and this 11 year old girl I think she is bossy. She is demanding, when kids try to mess with her, she backs them off, she isn't afraid to speak her mind and she can sound harsh sometimes. She is kind and caring but dont mess with her kind of girl. Some students find her very difficult. The teacher told me, "She may be a bit difficult but she knows what she wants and she gets it. It's a good thing, ESPECIALLY for girls."
I told her mom she was acting like a brat and a little demanding. Her mom says, "Excuse me? You have no right to say that to my daughter. She isn't any of those things. She is assertive and tough girl. That's fine because she will be successful in the future. That's how I want her to be. She has a sense of self and knows what she wants but I still gotta guide her. I rather have her be an assertive tough, strong girl than a too nice, quiet, timid girl. She gets her attitude from me which is a good thing. Leave my daughter alone!" I gotta work with these kids again today. :(