Wrong.. my DD14 made the decision when she was 12 (and younger) that she was ok with a C, especially if she didn't have to study. She was very ok with getting an F in an assignment knowing she would still pass the class.
And no, she isn't/wasn't depressed. She was smart enough to know what she wanted and what the consequences of her actions were. It drove me nuts (didn't bug DH, cause he is a mini him

). She would always do well on the first report card, and go downhill fast. She has ADD and has a 504, so the books came home and we had them. Our biggest issue was HW. Teachers were suppsoed to tell us after 2 or 3 missing assignments, so we could get her to catch up before she became overwhelmed. OY!!
She is now in HS and so far so good. Maturity will help. We tried rewards. We have a brand new cherry apple red Fender Electric guitar, that was purchased 2 years ago. She hasn't earned it yet! I am hoping this year..we shall see what happens.
Its very frustrating...DD is so so smart (she doesn't think she is) and has so much potential. We have also drummed into her that teachers in HS will not be nice like in previous years.
YOu can try rewards..but she has to realized the consequences. Talk to teachers about her assignments and try to stay on top of it. But if she wants to fail, she will, and she has to realize what is going to happen if she does.
Good luck...it does get better!