If two parties have already agreed that one will perform a certain amount of work and another will pay a certain amount of money, it is unreasonable for either side to demand more than what the other side agreed to.
A contract becomes meaningless if one side can arbitrarily adjust the terms.
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.
That's not good for anyone.
Including software. It is also one of those reasons why the 40 hour work week is mostly a myth.
The only effective (read: doesn't cost anything to execute) strategy employers have to calibrate how much workload can be achieved in a given time is to keep increasing the amount of work until people start quitting or jobs go undone, then scale back a little bit. There are other strategies that cost more to execute or require enlightened management; but they aren't the norm in my experience.
All a teacher can really do for a class is either 1:1 time with an individual or N:1 time with a group. All the details of exactly what gets done only obscures the fact that there is always going to be a good outcome for students if the teacher puts in another half-hour of unpaid work and that teachers shouldn't do that because it is unpaid.
The general idea: https://en.wikipedia.org/wiki/Work-to-rule
Some examples, many of which only do "work-to-rule" on certain days, not even on all of the days. https://chicago.cbslocal.com/2018/02/09/west-chicago-teacher... https://www.orlandoweekly.com/Blogs/archives/2018/01/26/oran... https://www.floridatoday.com/story/news/2018/01/04/unhappy-r... https://www.tulsaworld.com/news/local/education/tulsa-public... https://www.washingtonpost.com/news/answer-sheet/wp/2012/11/...