HAHAHA
My girlfriend is a second year high school teacher. You are technically right that she is required to be on campus for the required 7 or 8 hours. But, there is nowhere near enough time to grade, plan, prepare, work individually with kids (makeup work mostly), etc, in that time. She puts in 12+ hour days. Everyday. Including weekends.
And over the summer, you are right that it is mostly free, except for several weeks you give up to do professional development, AP course training, and any schoolwork to move up the salary schedule a bit.
Her salary schedules are here [1]. With a masters degree, she would top out at $58k/year after 20 years on the job (assuming the masters track). Starting salaries are in the $35k range.
Meanwhile, I am a computer programmer, working a legit 8 hour a day job, making much more than she is now.
What annoys me more than just teacher pay though is the other spending in the school. She was pulled in to teach english this year. When the english department met to work out the plan, they had to blow their whole budget for the year on a single class-set of a paperback. (30ish copies). That's it. No more books, no miscellaneous DVDs / Documentaries purchased. That's it. Well, except for when she just goes and pays for it. Which she did last year to the tune of $350ish.
[1] http://thompsonschools.org/cms/lib07/CO01900772/Centricity/D...