I am in a similar situation. I have a 10 year old sixth grader and she blatnatly ignores just about everything I tell her to do. It's not like I am asking her to do anything difficult, just a few simple chores, making sure she keeps up with her homework, that kind of thing. Most of the time I just end up yelling at her, to which she turns around and says, "Why do you yell at me all the time?" Unfortunately, my younger child is beginning to follow her lead. Ugh!
She's getting your goat, and she knows it. You should consider remaining calm at all times, and making her earn her privileges through good behavior. Don't get angry; instead, don't reward her negative behaviors, and give an incentive to correct them.
