TR,
I do think the kids should have spoken with both the chorus director and their coach as soon as the conflict became known to them. I also know that teenagers frequently duck this kind of responsibility, in part because they hope that the adults will handle the difficult decisions for them if the kids just wait it out. No matter how this plays out, I think the coach should use the opportunity to teach the kids about accepting responsibility.
Actually, if this really is a case of two school-sponsored activities conflicting with each other, then I think the school should have a policy to handle this, without repercussions to the kids. I really don't see how either activity can claim priority, nor do I think that kids should be required to take part in only one activity just to avoid this kind of conflict.
If the chorus is an actual for-credit class, then the priority was already set when the school scheduled the New Orleans trip.