I know my son lost alot of motivation at the age of 13. I was going thru chemo then and had no energy to fight his battle with an algebra teacher. His grades fell. 69 is a F at his old school. So he went from B to F. Put him out of the challenge program.
I don't know how to get them motivated again. Sometimes we have to listen closely and maybe even find out something they really like. Maybe some type of nice reward for improvement. Clothes are a good reward for teen girls. I hate to sound like primary school where rewards are the norm to get good behavior but maybe it could help.
Unfortunately those bad grades in high school are really hard to overcome and the freshman year is very important. Talk to a counselor at school. Maybe if you show interest at school this will help her not to get labeled by teachers and administration as a bad kid. Sometimes an amazing teacher can turn it around and change a kid's life. See what the school can do for you. Some are there all summer. But don't tell her as she may resent that you are going on her territory.