This study compares middle schools to K-8 schools, as well as to newly formed K-8 schools that are part of a K-8 conversion policy. The outcome is student achievement, and our sample includes 40,883 eighth-grade students from 95 schools across five cohorts. The analysis uses multilevel modeling to account for student, cohort, and school-level variation, and it includes statistical controls for both population demographics and school characteristics. The results find that older K-8 schools perform significantly better than middle schools, and this advantage is explained by differing student and teacher populations, average grade size, and school transition. Newer K-8 schools did not enjoy the same advantage despite having smaller grades and lower transition rates, due to their more disadvantaged populations. (Contains 4 tables and 3 figures.)