She knows something that the student doesn't.
If she plays as well as the teacher plays... she'll wind up a teacher. Not an orchestral star. A run of the mill high school orchestra teacher. The pay is decent, the hours long, the glory non-existent. It's a place for the experienced, but untalented, unlucky, undriven. As far as careers for musicians go, it's a pretty steady gig, but it's not a career in music. It's a career in education. You won't sell albums. You won't play shows.
The girl was dreaming to be a high-school orchestra member, and that's where the dream ended. She would have been a happy high school student, but a miserable adult.