I don't know what to do about my daughter

I'm hoping some parents reading this have been through this and can give me an idea that might work.

My daughter is a sophomore in high school.

Her entire school career, she has been an over-achiever. She had straight A's (4.0-4.5 GPA) every single year. She skipped the 6th grade because she was so far past every other student. Even after that, she maintained straight A's. She was so competetive about her grades that she would do extra credit work to get a higher A than another student. In 7th grade, she decided that she was going to graduate from high school Valedictorian, she was going to go to a University, graduate with a 4.0, then go to Harvard Law School. Her long-term goal was to be a Supreme Court Justice. Any other kid, I would have just said, "Yeah, ok sweetheart" not really thinking she would do it. But having seen her drive and ambition her entire life, I was absolutely positive she could (and would) do it. She even sent a letter to Harvard admissions office, telling them her plans and asking what she could do to increase her chances of being accepted into their school.

Then, in the 9th grade, it all changed. Last year she just stopped trying. She didn't turn in her work. She scored badly on tests. She failed nearly every single class. I thought it was just her adjusting to High School, but this year has been just as bad. We grounded her in November, right before Thanksgiving. We told her that, if she had at least C's and above in every single class at the next grading period (which was right after Christmas), she could be off grounding. She didn't do it. Long story short, she has been grounded since November. She is lying about doing her work. We're finding out from her teachers that she isn't doing assignments that she said she was doing. I even tried sending a print out of her weekly work (from the web site where her grades and assignments are listed) to school with her, and having her teachers initial every missing assignment as she turned it in to make sure she was catching up. I found out later that the teachers weren't even seeing them. I don't know if she was forging their signature, or what. She isn't getting into any trouble. Her attitude is fine. This last grading period (about 3 weeks ago) she had mostly A's, B's and C's - her best report card since 8th grade - but she still had 2 F's. Since we could see improvement, we let her go back to youth group. Not off grounding, just youth group. I just checked her grades tonight, she is failing every single class again. She has missing assignments. We tried to talk to her about it and she just cried, saying she's doing the best she can. But with missing assignments, I don't believe that.

I just don't know what to do. Two years ago, she wanted to go to Harvard. Now, she may not even graduate High School on time.

We've tried talking, disciplining, communicating with the teachers (which is a whole separate frustration), staying on top of her every minute, and finally giving in a little. Nothing is working.

I KNOW she is smarter than this. She is being lazy. When she does her work, she gets good grades. The biggest problem is her simply not turning in assignments.

What do we do?