Ask her how she feels about her grades this term. More importantly, ask her opinion of the courses themselves -- are they challenging and well taught, etc. Which courses are her favorites? Why? In the course of open-ended, non-judgmental discussions like this, her assessment of her grades and what she thinks she'll have to put forth academically will probably surface. If you indicate that poor grades are problems to be worked on rather than "the end of the world," she will be far more encouraged to think about how to improve them.
Remember that she will probably bring home a variety of grades throughout her high school career. How you respond to them and to her will have a significant effect on her school achievement. Paying attention to other parts of her life will let her know that you view her as much more than just a kid who should be getting all A's. She should never be made to feel that she is valued only if she always lives up to your sense of her academic potential. Thanks for writing.