This chapter takes a closer look at the out-of-school factors that play a critical role in understanding student achievement on TIMSS & PIRLS 2011 in Germany. Specifically, we introduce the background of empirical studies on the relationship between school composition and student achievement. We argue that, despite the limitations of descriptive data and difficulties of making causal inference, findings from national and cross-national studies are consistent with the existence of non-educational “effects” on student outcomes in school. The question is just how large this relationship is in Germany. To answer this question, we first describe heterogeneity in school composition across schools within Germany. We next revisit the findings in Germany from TIMSS & PIRLS’s Relationships Among Reading, Mathematics, and Science achievement at the fourth grade: implications for early learning (Mullis, Martin, & Foy, 2013), first describing the statistical models used and the results reported. We then specify new hierarchical models that take include a more robust set of school-level student background characteristics in order to give greater insight into how school composition varies across schools in Germany and, in so doing, that can help to explain with greater nuance the relationship between school composition and student achievement.