This. Kids aren't just little adults. And I see it as our job to guide them, not punish them for not being fully adult...though even adults fall victim to the sunk cost fallacy. How many people stay in broken marriages or careers they hate because they've already put so much into them? Heck, how many people are in those careers in the first place because they're living their parents' dream for them? But regardless, this could have been a teaching moment. End the lesson, tell the kid she needs to pay for it since it was wasted time, and then move on to a discussion about more appropriate ways of demonstrating her desire to stop. In the great scheme of things, the kid CROSSED HER LEGS. She didn't punch the teacher, break the piano, or run out of the room screaming. Sure it's not the appropriate way to handle things. But it's hardly a federal crime.