I understand your point. It's much more convenient to play a cello when you wear a pant. But you feel that it is a bit too casual to wear a set of sports outfit in such an occassion.
What I think is ...
Let your daughter wear the sports wear (the sports pant and sweater). Instead of wearing the sports jacket on top, wear the 校褸. But if your find wearing a 校褸 is too bulky to play the cello, wear a wool vest (冷背心). Your daughter will look smart!
One more thing, clean sports shoes, please!