About the only teachers that work any significant about beyond the 6-7 hour school day are teachers that must grade essays. So, your myth is already busted.
I teach physics. There are some problems with the statement I put in italics above. I recognize that the facts vary from district to district, but I have also never met a teacher in any district that had a regular 6 or 7 hour day.
Our contracted day is 8.5 hours long, which includes one 22 minute lunch. Technically, I'm finished at 3:45. Almost every day of the week, I am there at least one hour late, often two. There are labs to plan and setup, students who need help, and meetings to attend. If I average an hour and a half of extra time at school, that's already 10 hours per day. I also take work home if I can't get it done after school because, for example, students come in needing help or reassessment. Perhaps on average an extra half hour per night.
If I average 10 hours a day at work and a half hour a day at home, that's about 1880 hours per academic year. That's 90% of the 2080 hours a normal 8 hr/day full time job.
There are also the other professional activities and duties I participate in, such as continuing education, networking with other science teachers and scientists, and keeping current on research in physics and education. I take classes and attend workshops and conferences during the summers. For example, I have spent about four hours per week researching and planning, plus five full days on-site at workshops this summer.
I'm not complaining, I just prefer that people take a more factual look at teaching careers, not the mythical "6 hour day part time job" that many people would have you believe.