My questions for you are:
Why make your child go to a prodigeous school if she does not want to go? Why does she HAVE to have above average grades, when "mediocre" grades can still take her to whatever career she wants in life? Why are you looking at all of the lack of achievements instead of what she has already achieved? Why are you not litsening when she says you are making her feel pressured and unhappy?
I do not mean to offend you by these questions, but I'm asking you this because I know from not only my own expirience but also from my older brother's that you cannot force a child to become motivated in school and get good grades. I say, first be thankful that she's passing, compliment her on her strong points, and don't put so much pressure on grades. From my expiriences, this doesn't motivate but only strains your relationship.
Let her be free to enjoy the social aspects of school, be patient as she tries to figure out what she wants to do with her life, and regardless of what kind of life she wants it is important to be supportive of it. After all, this is HER life and HER decisions to do what she wants with it not matter what you want for her. It is at this point that motivation comes naturaly.
Will she fall down? Of course.
Will she make a bad choice here or there? Most likely.
But it is important to let your child fall once in awhile so they learn how to not trip themselves up.
Once again, I'm not meaning to offend any parent by this post.
And no, I'm not some sort of child expert.
But I'm talking from my own expiriences here, as well as my brother's.
Believe it or not, things could be FAR worse, both my brother and step-brother would be considered gifted...or at least very bright, and not only did they fail school, but I swear they failed school with the lowest grades you could possibly fail with. I never considered myself smart, but I must be considering I scored on average in the 95th percentile on the pre-ACT exam. I've only managed to pass school with decent grades (though like your daughter it's far less then my actual potential) because I feel bad for my teachers when I don't hand in my work because I know they worry about me.