In most states the average teacher's salary is in the mid $40k range. And if you divide that into the 12 months of the year, it isn't great money. Yes, they have good health benefits, but plenty of gov't jobs have that.
Only 14 states have teachers averaging above $50k a year. While 28 states have an average teacher's salary of less than $45k a year.
According to:
http://www.ehow.com/about_5212740_average-middle-class-salary-range_.html
"The 2008 census reported the medium income as $50,233. The PewResearch Center suggests that the middle income range is 75 percent to 150 percent of the median income. This would make the middle class income range $37,675 to $75,350."
Several states have average teacher's salaries that are only in the "middle class" by a couple of thousand dollars. South Dakota's average teaching salary is below the middle class line.