Well, school bands/orchestras, particularly elementary ones, can be very low level. It is not surprisng that your DD, with some instrumental training in piano, may not need to practice the school music. If that is the case, then it would be cruel to force her. At this point, DD does not practice either her 7th grade music or the honor music while DS only rarely practices his honor music. I don't pressure them to do unnecessary practice but they both practice music for their private teachers otherwise they would look like idiots during their lessons. For DD, that is not enough to fill the required school practice charts so she plays her own music. There was a time period when DD was particularly resistant to practice but I found out it was partly because she didn't have any music to motivate her. Once we bought a ton of popular music that she chose, she was more motivated to practice.
As for missing the train, it all depends on your middle school's policies. For our district, it would be difficult, but not impossible to pick up an instrument in middle school. The expectation is that students played a year in their elementary bands/orchestras but a motivated student can audition to show that she meets the minimum expectations.
