(By Jahi Chikwendiu/The Washington Post) (By Jahi Chikwendiu/The Washington Post)

Every now and then someone in education policy (Arne Duncan) or education philanthropy (Bill Gates) or the media (Malcolm Gladwell) will say something about why class size isn’t really very important because a great teacher can handle a boatload of kids.

Not really.

A new review of the major research that has been conducted on class size by Northwestern University Associate Professor Diane Whitmore Schanzenbach and published by the National Education Policy Center at the University of Colorado Boulder makes clear that class size matters, and it matters a lot. Schanzanbach, an associate professor of human development and social policy at Northwestern and chair of the Institute for Policy Research’s Program on Child, Adolescent, and Family Studies, writes in the review:

Considering the body of research as a whole, the following policy recommendations emerge:

*Class size is an important determinant of student outcomes, and one that can be directly determined by policy. All else being equal, increasing class sizes will harm student outcomes.

* The evidence suggests that increasing class size will harm not only children’s test scores in the short run, but also their long-run human capital formation. Money saved today by increasing class sizes will result in more substantial social and educational costs in the future.

* The payoff from class-size reduction is greater for low-income and minority children, while any increases in class size will likely be most harmful to these populations.

* Policymakers should carefully weigh the efficacy of class-size policy against other potential uses of funds. While lower class size has a demonstrable cost, it may prove the more cost-effective policy overall.

Why do small classes work? She writes:

The mechanisms at work linking small classes to higher achievement include a mixture of higher levels of student engagement, increased time on task, and the opportunity small classes provide for high-quality teachers to better tailor their instruction to the students in the class.

The research is there. Class size matters. Even the finest teachers are limited in what they can do when they have large classes.

So can we stop pretending that class size doesn’t matter?