A teacher in South Dakota with a bachelor's degree and 10 years of experience earns $33,600 per year, which is less than the average auto repair worker. This grievance against salary injustice is nothing new, of course, but this particular example comes to us from a new national study by the Center for American Progress, which details the chicken feed teachers are forced to subsist on as they altruistically keep your hopeless children literate.

Teachers are underpaid. In politics and also in everyday life, this is almost universally accepted. Everyone admires teachers. Everyone wants good teachers for their children. And naturally, liberals believe that contrasting these salaries will emphasize the irrationality and unfairness of the marketplace.

But it doesn't. And the first and most obvious reason it doesn't is that teachers actually do quite well for themselves when you consider the economic realities of their profession.

A 2012 study conducted by The Heritage Foundation found that workers who switched from private employment to teaching most often took an hourly pay increase, whereas most of those who left teaching for the private sector took pay decreases. More specifically, a few years back, using Bureau of Labor Statistics and National Compensation Survey numbers, the Manhattan Institute looked at the hourly pay of public-school teachers in the top 66 metropolitan areas in the country. It found that teachers pulled in about $34.06 per hour. Journalists, who have the vital job of protecting American democracy, earned 24 percent less. Architects, 11 percent less. Psychologists, 9 percent less. Chemists, 5 percent less.

It's also worth asking what an average auto mechanic might be willing to give up for the security of tenure. What would a guaranteed pension and a lifetime of health care be worth to a plumber? Considering how hard unions fight to keep these things, I imagine they're worth quite a bit.

Then there is the matter of demand—or lack of it. According to Andrew Coulson at the Cato Institute, since 1970 the public-school workforce has roughly doubled, from 3.3 million to 6.4 million (predominately teachers), while over the same period, the enrollment of children rose by only 8.5 percen—or a rate that was 11 times slower. Recently, the National Council on Teacher Quality found that schools are training twice as many elementary-school teachers as they need every year.

With this kind of surplus, the question we really should be asking is: How are teachers' salaries so high?

The second and less obvious problem with the mechanic-teacher comparison is the snobbish suggestion— thrown around by teachers unions and their allies all the time—that working with your hands is less meaningful or valuable to society than working with kids.

Now auto technicians make an average of $35,790 nationally, with 10 percent of them earning more than $59,590, according to BLS data. According to a number of experts from large car companies, there will be a serious shortage of mechanics in the near future, as demand is expected to grow 17 percent from 2010 to 2020. That's 848,200 jobs, according to USA Today. And judging from the information, mechanics are asked to learn increasingly high-tech skills to be effective at their jobs. It wouldn't be surprising if their salaries soon outpaced those of teachers.

"The bottom line," says the Center for American Progress, "is that mid- and late-career teachers are not earning what they deserve, nor are they able to gain the salaries that support a middle-class existence."

Alas, neither liberal think tanks nor explainer sites have the capacity to determine the worth of human capital. And contrasting the pay of a person who has a predetermined government salary with the pay earned by someone in a competitive marketplace tells us little. Public-school teachers' compensation is determined by contracts negotiated long before many of them even decided to teach. These contracts hurt the earning potential of good teachers and undermine the education system. And it has nothing to do with what anyone "deserves."

So if teachers believe they aren't making what they're worth—and they may well be right about that—let's free them from union constraints and let them find out what the job market has to offer. Until then, we can't really know. Because a bachelor's degree isn't a dispensation from the vagaries of economic reality. And teaching isn't the first step toward sainthood. Regardless of what you've heard.