I'm with some of the other mothers who gave examples of punishments you can push on her. If I were in your shoes, I would sit down with her, you and your husband, and have a sheet of paper already prepared beforehand by you guys. I would start out by telling her a few simple statistics, like "___people out of 100 (easier for kid to understand than using %) that do not go to college make ___much less money a year than people who do." Or maybe say something about her current grades, and what types of colleges she will have as her choices, whereas all the other kids in her class that have above average grades can pick from ____. Then, I would say "Since you have proven to us that we cannot trust you with being responsible for your grades on your own, the following things are going to happen until you prove us otherwise.
Then I would have a list.....something like
Everyday you will bring home EVERY book from ALL your classes.
Everyday you will have all your teachers sign a paper that your father and I will prepare and send with you, stating that you handed in your homework.
Every night during the week, you will come home and work on your homework until done, then you will spend 1 hour studying and reading from your textbooks.
On Saturdays, we want to see every paper, every test, and every worksheet handed back to you from your teachers from the whole week. (have a place in her room or something where she can keep these throughout the week as she gets them)
Until your grades are up, you may only get together with your friends one night a month, or two, or whatever.
Then, I would have very specific consequences spelled out for her if she fails to do any of the things you decide to demand from her.
If you don't do this, you're going to be totally responsible when she is trying to get into colleges and being denied. Plus, she's only going to feel worse about herself when she is truly trying, but can't make it in anywhere she really wants to.
If the things on your list don't make much of a difference, than I would change them up and try again. Eventually, you will find things that she really really hates, and THAT's when you get results. I think that bringing all books home everyday is such a good one, that would really suck! Plus, sitting with you guys while you go through her papers, that would suck if she has any bad grades in there!!! Stubborn or not, there's ALWAYS a way!!