Background/Context: It is well established in the research literature that socioeconomically disadvantaged students and schools do less well on standardized measures of academic achievement compared with their more advantaged peers. Although studies in numerous countries have shown that the socioeconomic profile of a school is strongly correlated with student outcomes, less is understood about how the relationship may vary if both individual student and school socioeconomic status (SES) are disaggregated. Purpose/Objective/Research Question/Focus of Study: This study examines the relationship between school SES and student outcomes in more detail by asking two research questions. First, how does the association vary for students of different socioeconomic backgrounds? In other words, is the association stronger for students from lower SES backgrounds than for students from higher SES backgrounds? Second, how does the association vary across schools with different socioeconomic compositions? In other words, are increases in school socioeconomic composition consistently associated with increases in student academic achievement? Population/Participants/Subjects: This study uses data from the Australian 2003 Programme for International Student Assessment (PISA). The sample includes over 320 secondary schools and more than 12,000 students from Australia. Research Design: This study is a secondary analysis of data from the Australian 2003 PISA. Descriptive statistics are used to compare the average reading, mathematics, and science achievement of secondary school students from different SES backgrounds in a variety of school SES contexts. Conclusions: The two main findings of the study are that increases in the mean SES of a school are associated with consistent increases in students’ academic achievement, and that this relationship is similar for all students regardless of their individual SES. In the Australian case, the socio-economic composition of the school matters greatly in terms of students’ academic performance.