I’m no doctor, so take this with lots of salt.
It sounds like a classic scenario. Very successful in elementary school, but she hit her limit in high school and is struggling. She being really frustrated, to me, sounds like she has a big heart. She’s a talented pianist.
Is she at all interested in the 4 classes or is this something she just has to take? One thing I’ve realized is I do not respond to external motivation. If I had to take a class in college but was not interested then I did poorly compared to the classes I had passion for. Everyone else got A’s in the easy electives and struggled with the hard classes but I was the opposite because I internalized the challenge of the hard classes. It was an emotional connection that motivated me. This is an important idea. Where that emotion came from, I don’t know. It might be that after struggling with K-12 I finally found something I could do so the bigger the challenge the better. It could also be that the mental gymnastics of solving hard problems I could get my head around was perfect for me.
So, is it possible that your expectations are obscuring her view of what she wants to do for a career? My mom, dad, and both my brothers all wanted the three of us siblings to be engineers. So I soaked that up and tried really hard. Turns out that it wasn’t too far off from what I really wanted to do but just enough to cause problems. What I really enjoyed was creating, anything. Or solving tough problems. I eventually got into software and that was a reasonable fit. But I’ve found problems in the strangest places that I’ve really enjoyed. Turns out I like working with children and teens as well. I’m not at all suggesting your granddaughter do these things but just that she has to find her thing.
When she plays piano does she get emotionally connected to the music? Does she see and feel the music or does she just play it? Does she enjoy it but feel that this is not an acceptable career in your eyes? or maybe her parents?
There are also articles on “good jobs for those with ADHD” on this and other websites.