I would say that there are different scenarios for different folks. From my perspective--teaching in a private university, having taught in a local Junior Highschool, and a cram school--I'd say that as the grade level goes up, the work becomes less administrative. This would mean that work besides lesson planning and actual teaching (putting reasearch on the side for now) becomes less. The work hours may not change so much, but the freedom to leave the premises of the institution become greater.
The teachers here where I work, tend to leave when they have done what they want to for that day. There is practically no pressure to stay until the first person leaves, to have an excuse to leave. There are basic office hours, but those are generally from 9:30 (plus/minus) to 5:30 (plus/minus).
In this scenario, I'd have to say that though the actual work done usually would not take all the non-class hours spent on the premises, the time basicaly required by the institution for teachers to be here, is not so long.
Other professions, other situations, I know. Maybe we'll hear more. (My brother-in-law has a middle management position at a local hospital and leaves earlier than I do, and always comes back later than I do.)