Several countries have expanded extended education in recent years. In Germany, the most substantial educational reform is the ongoing transformation of the traditional half-day school system into an all-day school system. Among politicians, expectations are high that all-day schools will promote student achievement and reduce social achievement inequalities. To test these assumptions, we used representative data from the National Educational Panel Study (NEPS) to estimate two-level latent growth models for achievement in grades 5, 7, and 9. The analyses revealed initial achievement differences but no differences in achievement growth or changes in inequality throughout secondary school. This suggests that selection mechanisms are at work but that half- and all-day schools are not differentially effective. We discuss these findings in light of the international debate on the quality of extended education.