President Obama, who can count the teachers union as one of his most reliable constituencies, rarely addresses the issue of unemployment without calling for hiring or rehiring more teachers.

Now, Andrew J. Coulson, director of the Cato Institute’s Center for Educational Freedom, puts forward an idea I have long suspected: America has too many teachers.

Writing in the Wall Street Journal, Coulson notes that the number of public-school employees has doubled in the last 40 years, while student enrollment over the same period has increased only 8.5 percent.

One argument for having more teachers is that smaller classes and more public education employees promote more learning, and therefore, since an educated workforce is essential in a sophisticated society, more teachers lead to more prosperity. But is this the case?

To find out if that's true, we can look at the "long-term trends" of 17-year-olds on the federal National Assessment of Educational Progress. These tests, first administered four decades ago, show stagnation in reading and math and a decline in science. Scores for black and Hispanic students have improved somewhat, but the scores of white students (still the majority) are flat overall, and large demographic gaps persist. Graduation rates have also stagnated or fallen. So a doubling in staff size and more than a doubling in cost have done little to improve academic outcomes.

Nor can the explosive growth in public-school hiring be attributed to federal spending on special education. According to the latest Census Bureau data, special ed teachers make up barely 5% of the K-12 work force.

The implication of these facts is clear: America's public schools have warehoused three million people in jobs that do little to improve student achievement—people who would be working productively in the private sector if that extra $210 billion were not taxed out of the economy each year.

Interestingly, private education is more efficient, producing better results at lower cost. School choice, not more school teachers, is the promising path for educating people. This is not what President Obama’s friends in the union want to hear.