I agree with Billythex, that the uwritten rules of JWs are inforced at the sole discretion of the Elders of a particular cong. Some Elders are more rigid than others and many have their own opinions which often influence whether they allow something or not.
Generally speaking, most elders allow band, chorus, sports, drama, etc. when it is offered as a school subject during school hours. When it is EXTRA Curricular (sp?) when done AFTER SCHOOL, they highly discourage and somtimes actually forbid. The reasoning is that the child is unecessilarily exposed to "bad associations",competion, patriotic and Holliday music, etc. Of course, the age-old his/her time is better spent in the minestry,studying BS usually comes into play as well.
When I was a JW kid in the '60s I was allowed to take Concert Choir as an elective in both middle and High school. The trouble for me came when the choir performed Christmas and Patriotic songs during the school assemblies. Not being able to participate severly hurt my grade and I ended up dropping out. The band performed at all the football games and because they are after school, the JW kids I knew didn't join.