No. Not my job. If a parent wanted to cut back their kids hours for low grades, I would absolutely be on board, but I'm not going to make that decision for them. I will applaud a child who comes to me thrilled with their grades, but also continue to support a child who is barely passing.
I say this as someone who struggled through school terribly and gymnastics was I place I could get away from that. My struggles in school were between my parents, teachers, and myself- I didn't need to include my coaches in that circle as well. At one point I had a coach ask for report cards, creating a rule that girls with grades below a certain level would have some sort of consequence that I don't recall, fortunately my parents were not on board and I did not need to show them. I just recall being mortified at the prospect. I felt badly enough about my school performance at the time.