Teachers are expected to work a certain number of hours per year - it's all written down. A fairly high % of that is the actual time in a classroom with kids. But then there's the lovely phrase "and other duties as directed". About 20% of the total expected work is given to planning, meetings, grading, parents' eve, spending 1-2-1 with kids needing help, talking to colleagues, going on training courses, etc etc. Obviously, the vast majority of teachers do WAY more than the expected number of hours, and are doing all of those things for free as the pay reflects the expected hours, not the actual hours.
So, every 'extra' thing, like a play or a trip or anything, is unpaid extra done for love. It also explains how the schools get around the ruling about having 11 hours between the end of one shift and the start of another - we 'volunteer' to be at the school play, clearing up until midnight, then back in school just a few hours later.
I did the maths once - if I'd balanced my working hours out over a 48 week year, rather than condensed into the school year - I'd work about 50 hours a week. But most of it gets condensed into termtime, which is why teachers moan so much about how hard they work!