This study examines inequality based on family structure within the school environment, focusing on South Korea's standardized education system. Previous research has pointed to potential challenges associated with a high concentration of students from single-parent families in schools. However, in the context of a standardized education system that aims to minimize variation across schools, these dynamics may manifest in different patterns. Using a nationwide administrative data set, the National Assessment of Educational Achievement, this study finds a significant interaction effect between the proportion of students from single-parent families within schools and the academic achievement, which is particularly pronounced for students from single-parent families. The results suggest that institutional contexts may play a significant role in reducing educational disparities associated with family structure, thereby providing schools pathways to mitigate the academic challenges prevalent among students from diverse family backgrounds.