When the United Auto Workers walk off the job, no one pretends that they are acting in the interests of car buyers. Everyone knows union members want better wages, benefits, working conditions and job protections, and going on strike is an effective way to exert pressure on employers.

The calculation is no different when teachers strike. Unions like the National Education Association, the American Federation of Teachers and their thousands of state and local affiliates exist for the same reason as the UAW: to advance the interests...