Since the passage of No Child Left Behind, schools have been looking for resources that are proven, through research, to improve student achievement. The purpose of this article is to determine if there is a relationship between class size and student achievement among 137 school systems in Tennessee. The authors provide a review of the literature on student achievement, including studies on the Student/Teacher Achievement Ratio (STAR) and the National Child Development Study (NCDS). Using a hierarchical regression analysis, the authors examined four achievement measures, controlling for the influence of socioeconomic background. The authors conclude that economic background has a greater influence on academic achievement of elementary school students in Tennessee than high school students; and that after controlling for socioeconomic status, the student-teacher ratio was not significantly related to the four achievement measures. (Contains 6 tables.)