Thinking about this later it occurs to me that as you say that the problem may be that she has fallen behind and her attitude is a defense mechanism.
I always hesitate to give advice as I have to say we seemed to get a lot wrong in the parenthood business but it is vital to keep the lines of communication open.
It is vital that there is trust between parent and child, my wife was always better at it than me but we always sat and ate together after the day and they were encouraged to discuss everything at the table between themselves and with us and we often swallowed our tongues at some of the stuff that came out. We used to say to each other that it was better we knew what was going on even if we did not like it.
I think that we don't really teach they why we are teaching what we teach as well as we could. Part of it is so you know how to do x or rather you will be able to relearn it faster when and if you need it and second it teaches you how to learn and gets you used to learning. Learning take actions, actions take practice. Also the why do I need to know this is a hard answer when the real answer is that you will be combining bits of everything you learn to do your job when we break it out into subjects.
You didn't post what income level the family has. We've taken our children through neighborhoods with low income families and showed them the difference between what we have and what they will have if they don't do well in school. That's not to say that college is right for everyone but we let them know that they are expected to do well. I wouldn't be too sad for my step-son, who is unsure about college, if he took up a trade that would allow him to support his family (when he has one) at a decent level. We've also let our kids know that we're not their fall back position if they don't give a good try. We're of the sort who would rather teach a man to fish rather than give the man a fish.
Beyond spelling out the economic facts and forcing a clear cause and effect on grades vs freedom / access to fun. I don't know what can be said. The problem is the message leaders they have are not talking about hard work = success nor about creating opportunities for success.
Some of what I have done is to take one of my sons to the fields where people were working picking lettuce, we parked there and I made him watch them work for a while.
I told him that if he wants to be uneducated he will wind up working stoop labor jobs that do not require an education, but they need fast hands and a strong back.
After this I took him to the farm owners house and asked if he would allow my son to work there free of charge so he may know what it is to have to live like this, the farmer said he would be willing. His grades came up shortly after.
Interesting. Could backfire.