Having been there, I'm afraid I'd side with Mumeee.........
All you can do is make it clear to her that she needs to keep her grades up. Once she gets to Uni, yes, she'll have to deal with it herself, but chances are, she will.
I have 12GCSES, above C.. 5 A Levels and 3 degrees (One undergrad, two postgrads).
The FIRST thing my mother said to me when I got the results of my BSc (I got a 2:1) was: If you'd worked harder, you could have got a first.
Well, YES......... I could have..... but I had a great time going out, making friends, meeting people (and these are also important elements of Uni life!)
While I appreciate that tertiary education is important, and seems to becoming moreso, there IS more to life.
As I said, I have more qualifications than you can shake a stick at, and I still struggle to get anyone to pay me more than £40K........
The only things I could really suggest is to try different types of learning - interactive, placement etc., and also look at the subjects she's doing - are there certain subjects she really excels at?
Just remember, she's 17.. I don't think any of us was totally motivated at that age!