Andrew Biggs writes at National Review Online about a persistent myth surrounding public school teachers.

According to the Economic Policy Institute (EPI) — a liberal think tank affiliated with teachers’ unions — public-school teachers earn, on average, 21.4 percent less than private-sector workers in other fields who have similar educational and demographic characteristics. According to EPI, by offering higher pay, public schools could attract and retain better teachers, leading to better test scores, higher graduation rates, and better jobs for the future. It’s a compelling argument, at least to the news media, who echo it in their coverage.

But if all that is true, why do private schools, even elite ones, pay teachers so little? …

… If paying higher teacher salaries would buy their kids a better chance, why don’t they demand an increase? In 2011–12, the most recent year for which data are available, the median full-time teacher in a non-religious private school earned a base salary of $38,000, 24 percent less than the $50,000 base salary for the median non-charter public-school teacher. Parochial-school teachers earned even less, at just $35,000 per year. …

… A better explanation for pay differences between public and private schools is that the much-touted 21 percent “wage penalty” is just plain wrong. As Jason Richwine and I have shown, the EPI pay-gap studies are technically flawed, producing absurd results for other occupations — such as that nurses and firefighters are highly overpaid, while telemarketers are underpaid. Moreover, of the hundreds of occupations that the EPI study compares with public-school teachers, it bizarrely excludes private-school teachers, conveniently shunting aside the occupation that is obviously most comparable.

But there’s a bigger story. For public-school teachers, large districts employing hundreds or thousands of teachers negotiate with a single teachers’ union, which itself may have secured a favorable position via political contributions and activism. This isn’t what you’d call a free and vibrant labor market.