It's not about one side arbitrarily adjusting the terms; it's about both sides realizing that they have a shared interest in keeping the organization running. It's just not feasible to write an employment contract that fully encompasses the work needing to get done.
Ensuring that a class gets taught well isn't just a matter of standing up in front of them from 8 to 3; any teacher can tell you about the amount of random surprise work that needs to get done at a school. You can call that work exploitation if you want, but the bottom line is still that someone has to do it.
