Could middle schools be bad for student learning? Could something as simple as changing the grade configuration of schools improve academic outcomes? That's what some educators have come to believe. States and school districts across the country are reevaluating the practice of educating young adolescents in stand-alone middle schools, which typically span grades 6 through 8 or 5 through 8, rather than keeping them in K-8 schools. The middle-school model began to be widely adopted almost 40 years ago. Now, reformers in Massachusetts, Pennsylvania, Ohio, Tennessee, Oklahoma, Maryland, and New York, and the large urban districts of Cincinnati, Cleveland, Philadelphia, and Baltimore, are challenging the notion that grouping students in the middle grades in their own school buildings is the right approach. For the last two decades, education researchers and developmental psychologists have been documenting changes in attitudes and motivation as children enter adolescence, changes that some hypothesize are exacerbated by middle-school curricula and practices. These findings are cause for concern, but there is reason to doubt their conclusions. Because the studies use data from a single school year to contrast students in middle schools and K-8 schools, most of the available research cannot reject the possibility that differences between the groups of students, rather than in the grade configuration of their schools, are actually responsible for the differences in behavior and achievement. To provide more rigorous evidence on the effect of middle schools on student achievement, the authors turned to a richly detailed administrative dataset from New York City that allowed them to follow students from grade 3 through grade 8. What they found bolsters the case for middle-school reform: in the specific year when students move to a middle school (or to a junior high), their academic achievement, as measured by standardized tests, falls substantially in both math and English relative to that of their counterparts who continue to attend a K-8 elementary school. What's more, their achievement continues to decline throughout middle school. This negative effect persists at least through 8th grade. The authors found that the middle-school achievement gap cannot be explained by a scarcity of financial resources for the schools. Instead, the cause is more likely to be related to other school characteristics, especially the fact that middle schools in New York City educate far more students in each grade. Although the authors' conclusions about the reasons for the middle-school gap are tentative, they are quite confident that the evidence shows that middle schools are not the best way to educate students--at least in places like New York City. (Contains 4 figures.)