Both Mayor Bloomberg and the City Council speaker, Gifford Miller, apparently having done the electoral arithmetic and concluded that the best interest of teachers unions and the false hopes of parents trump the substance of education reform, are going into this election in support of one of the discredited ideas that smaller class sizes are a key to educational performance. The mayor, in his executive budget last week, proposed spending $10 million to reduce class sizes from kindergarten through third grade. Mr. Miller, who is running for mayor, jumped on the bandwagon on Tuesday with his own “17 Seats” initiative to reduce classes all the way through middle school.

It’s too bad the bandwagon isn’t going anywhere. There’s no convincing evidence that reducing class size improves student performance in any but the most extreme cases, where class sizes are more than twice the New York City average or in special education. Proponents of class size reduction like to point to several experiments over the past two decades that purport to show that smaller classes work. But a careful look at these studies discloses methodological problems that raise questions about the results. And an international study from the Organization for Economic Cooperation and Development last year couldn’t find any meaningful connection between class size and student performance.

If anything, cutting class size may actually hurt achievement. California rolled out sweeping class-size reductions in the mid 1990s. Golden State test scores have improved somewhat since then, but the gains have been uneven and aren’t related to changes in class size. That’s because to reduce class size, you have to hire more teachers. One 2002 study from the Public Policy Institute of California found that in the rush to do so, districts had to scrape the bottom of the barrel to find enough warm bodies to stand in front of classrooms. New York would be under the same pressures. By the City Council’s count, Mr. Miller’s “17 Seats” plan would require 2,035 new teachers by fall 2006. No one has yet publicly stated how many new teachers Mr. Bloomberg’s plan would take.

Even if they aren’t competent at teaching, those bodies can still join teachers unions. That helps explain why unions are so supportive of class-size reduction, particularly if a debate about class sizes distracts from the more important discussion about teacher quality, a real determinant of educational success. The fact remains that class sizes have been steadily decreasing for the past half century, even as educational quality has also been on a downward trend.

Messrs. Bloomberg and Miller aren’t just trying to appease teachers unions; class-size reduction is popular with a lot of parents. That’s understandable. Some parents might find the idea of little Johnny being one of only 17 children in a class more appealing than the idea of Johnny sharing a teacher with 21 other kindergartners. Others might be more focused on the qualifications of the teacher. Well, why not let them choose, with a voucher that allows them to select a school based on class size, teacher quality, or any other factor they think is important? We don’t have a problem with small classes. Politicians who falsely present class-size reduction as a panacea are another matter. They may buy off the teachers union, but there’s no reason to believe either of these expensive plans will do anything for the vast majority of students.