Much of the case for reducing class sizes rides on the findings of the famed Tennessee Project STAR experiment, which found that student achievement significantly improved in the early elementary grades when class sizes were limited to around 15 students. Years after its conclusion, researchers are still parsing the data from this respected, longitudinal study of 79 schools in 42 districts, one of the rare studies to pull off random assignment of both teachers and students from kindergarten through third grade.

It turns out that the positive results from Project STAR, however, were based on comparing the average levels of achievement reported for both smaller and larger classes. But do all students benefit equally from smaller classes? A new analysis of the Project STAR data has found that it is the higher-achieving students who benefit from reductions in class sizes, with much less of an impact on their lower-achieving peers.

This finding presents yet another reason for states, districts and schools to reconsider investing their limited resources into reducing class sizes. While smaller class sizes may raise student achievement in the early grades, this study calls into question their application in reducing the achievement gap. Class size initiatives just might be helping the rich get richer.