Please keep us posted! And I don't believe that any school district would be permitted to prevent a student with Juvenile RA from participating in sports or anything else, at least not on that basis alone, because it's a violation of the ADA; that's not to say that they couldn't come up with another reason to make things difficult for her. Luckily she is young and advances in treatment are coming along rapidly so chances are excellent that even should your D be confirmed as having RA, in all likelihood she can continue with her music and the movement will help keep her joints active.
There is plenty of time in the future to tell a studio teacher about health issues, if necessary. Again, after acceptance would be ideal. She should definitely learn how to advocate for herself- a skill that serves everyone well!