Meemee, I get it. I let my kids do things of their choosing, with the understanding that it's going to end when it's time to move on to the next activity, whether you're engrossed in the first activity or no.
(IE-You can watch TV until it's time to leave for chorus, and then you will have to turn it off. No, I don't care if
(a) it's in the middle of a show,
(b) you *just* started a new show,
(c) you just decided you don't like chorus or
(d) you like chorus but want to skip it for today.)
DD does like chorus, but she hates tearing herself away from whatever she's doing beforehand. I read the Opera thing similarly. You tell them they are going to a particular event, and whatever activity they do beforehand doesn't change the original plans.
Now, on the class, I think it's fine to require her to finish out until the break and then decide how she feels about continuing. I am more goal and academic-oriented than many people I know though, and I know a lot of adults who don't take commitments or promises as seriously as I do. I think commitment is an important thing to learn. It's not that life does or does not revolve around them, it's that once you make a commitment to something, you see it through.