Yeah, as I say, I'm not saying I'm correct, just that it appears to be that teachers have an easy life. Though I guess it heavily depends on how good a teacher you are and how much effort you put in.
If you want to do tailored lessons, then yeh, I can see it being a massive chore. My experience on the other hand was working through text books and being taught the same stuff that's been taught for the past however many years unless there is a huge change to the syllabus. (Of course that is until Ofsted visits, at which point we get a customised lesson.)
Peer marking usually took forever because of the problems you point out. We'd be given the answer, and then spend 10 minutes while every student asks whether or not the answer / method their peer wrote was correct.