I think that's the difference between a good teacher and an average one. My wife was a teacher (left because of the poor salary). She worked 12+ hours a day five days a week (we made sure to keep weekends open but we were there on weekends too sometimes). You could see which other teachers were there 12+ hours. It was the ones that sent the most prepared kids to her class. There was an obvious quality difference between the kids who had teachers that worked 12+ hours a day and those that didn't.

Depends on the grade and subject. Some teachers have a whole lot more grading to do than others. And some grading or planning is slower-going than others. Also some schools are better about keeping time clear during the day for teachers to plan & grade than others.