Rising junior D was adamant about quitting orchestra (after 7 years). She couldn’t fit in all the other classes she wanted, and her schedule next year is tough. She was in the symphonic orchestra as a Soph. and felt she was one of the weakest in her section. She was stressed/crying about it and wanted to drop at the end of first semester this year. I talked her into finishing the year (couldn’t get into any class she wanted at the semester anyway.) Now the director got her to reconsider when he found she wasn’t signed up for next year. Last week of school she met with counselor about her schedule and decided to continue orchestra and move/skip a couple other classes. Director reassured her that she is a strong musician and needed next year-- and maybe he was a little rough on her first semester, but she did improve a lot. After being a little disappointed at first, I was fine with her quitting, and sort of surprised she reconsidered. (One of her best friends is in her section. . .that probably has more to do with it than anything.) A few years back, her older brother (a talented pianist) quit lessons/competitions his senior year. He told me a few years later that he regretted it. (Unfortunately we moved twice while he was in high school and he didn’t like his new teacher after we moved the 2nd time.) He still plays, though.
Band is a class and it is also an EC. She will take an Art class…but what will she do with that time?
For college apps, I would frame it not as quitting band but as taking up Theater/new job/etc.
An update on the situation. D did drop band for next year. Met with her counselor yesterday. During this meeting D learned that she was going to have a major schedule conflict for Senior year that she would have been learning about soon. She would not have been able to take AP psych next year as it is offered same hour as her band. So in dropping band, she was able to pick up AP psych AND microbiology. I guess overall it is a good thing. She really wanted the AP psych class as any major she is contemplating for college will involve a psych component. She is at peace with her decision and is now really looking forward to her final year 
Sounds like things worked out for the best. You can always ask the GC to mention that she had to drop band to take the classes and maybe she can find another more pleasant outlet to continue with her music (ex. lessons, a community group etc.).
^^ Agree with happy1 that the college counselor may mention this in his/her letter to colleges.
S quit playing baseball senior year in order to be in the musical. He had always loved singing/acting and the director had asked him to try out for three years. Senior year he asked him, again, to try out. He told him if he liked his part, he would be thrilled for him to participate. If he didn’t like his part, he would understand if he chose baseball. He was thrilled with his part, it was a rainy spring and many baseball games were postponed or cancelled, and his favorite teammates had graduated the year before. I missed baseball more than he did. I loved the musical and he was (proud mom here) fantastic. He never regretted his decision.