I'm just an undergrad, but I'll share some of my observations.
First, high school teachers and professors can have vastly different job descriptions.
For example, I go to a big research university and the professors there do research. Most of them teach only 2-3 classes a year, sometimes more sometimes less (I'm guessing the higher ranked profs teach less.) This is about 6 hours of classroom time, and throw in 2-3 office hours and a few hours for planning, and total working time associated with teaching is about 15.
Now, at the other extreme, take the community college I transferred from. I think most of the profs were only part-time and did other stuff for a full-time job. For example, maybe a guy had a Master's Degree in History and so he taught High School history and a couple courses at the community college every semester. The full-time profs usually taught about 4-5 classes per semester. Clearly, the community college profs were teaching much more than the profs at my current research university.
So, it seems that the research university people have it a lot better. However, to be a prof. like this, you have to do a lot of research (so it seems.) So, you might have to teach less, but you are expected to do a lot of research to keep your job (Erdos once quipped "A theorem a day gets you tenure and pay, a theorem a year you're out on your ear.") But, this isn't all that bad since you are doing research in something that you actually like to do. You got a Ph.D. in some subject that you love, and then you get a job where you "have" to do that subject. So, your teaching load is low at a big university (and I'm sure there is some continuum between these two extremes) but you "have" to do research, which probably brings your week up to 40 hours or so. But at the same time, the research that you "have" to do is more like getting paid to pursue a hobby.
Additionally, as Paul Halmos wrote, it is really important to become in involved in your academic community (e.g. referee journals, guest editor of journals, host/give conferences, etc) and I guess this stuff would either displace some research or just add a little time.
Either way, it seems to me that professors have a rather high level of job satisfaction.
As I said, I'm just an undergrad, so I could be way off, here. These are just my observations as an undergrad.