The massive power imbalance between employer and employee is the main factor here and the driving motivation for labor unions.

So much more than that even, so many are teachers must feel compelled to do what they must for the sake of their students. I know it's incidental, but the extra effort I see my own kids' teachers putting in without financial incentive is incredible. Education is a business where a lot of "employees" probably aren't doing the calculations around their pay-per-hour but rather are focused on the accomplishing the things necessary for their students success.

Maybe that's a long winded way of saying that I suspect a lot of teachers are bleeding hearts and do not closely manager their time or monetary budgets closely.

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.

If you are not sure in advance what you are going to want teachers to do, then this might just be an excuse to exploit them.

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.

Again: you're asking a teacher to cover roles other than teaching.

That's not good for anyone.

Feasible for whom?

For anyone, which is why work to rule strikes are such a well-known concept. Nobody in any industry can do it.

It represents the dichotomy between contracted work and what it actually take to do the job.

Remove 'contracted' and it applies to any work, or any day job in general.

Including software. It is also one of those reasons why the 40 hour work week is mostly a myth.