It may not work for this concert, but in future concerts, when giving the information about the event, you can tell families to stay for the whole show. For example, my band concert tomorrow is followed by a drama and dance performance. I stress to my students and parents how these other groups have worked just as hard as they have and it’s important to be respectful of your peers who just sat through your part of the show. I also am a meanie and make all families sign a letter stating that the student will attend and in that letter I ask for families to stay the whole time. However, nearly every show has a family who must leave right after the band’s performance for various reasons. If they tell me ahead of time, it’s okay and we talk about how they need to put away their stuff quietly and not be a distraction. I think you should also determine if this is the parent or the student. If the student wants to stay but the parent is forcing them to leave, that’s a little bit more of a sticky situation because you don’t really want to punish the kid. Anyway, it may be too late to enforce anything this time around, but you’ll be ready at the next show!