This paper examines changes in demographic and socioeconomic inequalities in student achievement over the school career, and the extent that these inequalities are accounted for by other influences such as, region and socioeconomic background (where appropriate), school differences and prior achievement. The data analysed are from a longitudinal cohort of Victorian government school students in Years 3, 5 and 7 between 2008 and 2012. The most important finding is the dominant influence of prior achievement which substantially reduces demographic and socioeconomic differences. The strong effects of prior achievement hold even after differences between schools and socioeconomic background have been taken into account. Therefore, policy positions and theories of student performance that give primacy to the socioeconomic resources of families when students are at school, or schools themselves, are not supported. The genesis of demographic and socioeconomic inequalities in student achievement occurs prior to Year 3 and point to the importance of factors operating in the preceding years.