Hi K.!
It's hard to watch our children struggle in school. I've been there years ago with one of my own four children, and I deal with it with the teens I work with. I'm a leading expert on teen girls (and their moms!)
The first step is to let your daughter know you are there for her, no matter what. This does not mean you condone or or agree with any negative behavior, but rather you are her ally in her struggles to turn her academic life around.
Second step is to find the real cause of her academic "decay." What does she need that she might not be getting in her life at the moment? Is school boring? Is she afraid to go to High School? Are drugs involved? Discovering what's true for her is vital to understanding how to help her turn things around.
The worst thing you can do is nag her, or bribe her to do better. Neither truly works on the root cause of the problem.
Learning to use "transformational listening" skills will help. In a nutshell, learn to allow her to speak her truth without making her wrong, interrupting, putting her down, telling her she shouldn't feel the way she does, or giving unasked for advice. Rather learn to hold her truth with respect, and ask geat questions out of curiosity, not spite, to help her find more of HER own truth as she answers. This is time to learn how to create a close bond, as the high school years are challenging for our teen girls.
I wish you both the best!
Dr. Jenn