Yes, they work 9 months out of the year, which leaves them 3 months to work another job if their teaching salaries aren't enough.
Here in the Cleveland area, the average teacher earns $54,000 per year. (This is according to a survey that the Plain Dealer published a couple of months ago.) According to the Manhattan Institute study, teachers here earn on average $38.86 per hour. That is pretty darn good no matter how you slice it.
If it's a Manhattan Institute study on teachers, it'll be missing a few things. Our HIGHEST (with PHD and additional credits) salaries are around $50,000 here. For hourly salaries, one might be using just teaching time. There is additional preparation time too (some of which may be included in CONTRACT TIME). Contract time is 40 hours a week here while teaching time is different. The OUTSIDE time is hard to factor in, but if it were to be factored in, the hourly salaries would be much lower.
The simple matter is teachers aren't dirt poor. BUT they aren't rich either. NO teacher at our school is the main wage earner in a home unless he/she is single. That is another thing to take into account--many teachers are co or secondary wage-earners. Nonetheless, I can't get much of a housing loan (if any) on my salary alone.
A simple measuring stick--if teachers are indeed paid so much, there'd be more males entering the profession (like me) and more would not leave for more lucrative positions.