Educators and policymakers are paying increased attention to the academic outcomes of students in the middle grades (i.e., Grades 6-8). One reform proposed to improve outcomes for these students is to replace middle schools (with Grade 6-8, 7-8, or 7-9 configurations) with K-8 schools. This longitudinal study evaluated the effects of continuously attending a K-8 school, rather than transitioning from an elementary school to a middle school, on Grade 8 reading and mathematics achievement. Drawing on nationally representative data from the Early Childhood Longitudinal Study-Kindergarten 1998 cohort ("N" = 8,237), the study used propensity score stratification to control for observable selection bias. Findings indicated that K-8 schools produce small, significant effects for reading (effect size = 0.15 or approximately 6-8 months of schooling), but nonsignificant effects for mathematics. Results were robust to several alternative specifications, including accounting for nesting of children within schools and using different approaches for propensity score matching. Findings provide conditional support for K-8 schools, highlight the need for cost-effectiveness research on this topic, and raise questions about the specific mechanisms for K-8 schools' advantages.