Mid-Quarter Time

Here we are a new school year. I had told Brit that it was a new year for her. She was able to start the year off with a clean slate. She was in control of how she preformed in her classes.
The first week of school she had come to me begging me to allow her to try out for the school play. I was very hesitant. I was looking at her last years grades and how she had put no effort in to her work. But I reminded myself that it was a new year. And I needed to let her prove herself. I let her try out and warned her that in the rules they had sent home with the sign up information that if she fails any of her classes she would be kicked out of the play.
Now, we are at the point where we have received some of her mid-quarter grades and she is failing 2 classes. It is not that she is having a hard time in these classes or that she does not understand what she is being taught it is she is not doing her homework. We ask her every night about homework.( When I look at her mid-quarters I see she is getting A's and B's on her tests and quizzes.) All we here is I did it in class or only for few classes. Now we get the mid -quarters. I had told here that if she fails she will not only be letting us down but the entire cast of the play. Now if at the end of the quarter if she has not improved her grades they will have to let her go. So then she needs to explain to not only her classmates why, but her family that she has invited to come and watch her. I was really hoping that the play would be a strong enough encouragement for her to want to do her work.
I am at a loss. I can only help her when she wants it.
Any suggestions out there. I have tried a lot. Taking things away. Phone gone, MP3 gone going out with friends gone. Nice new clothing from stores she likes, gone. You name it I have tried.