Declines in the scores of Australian 15-year-old students from the Programme for International Student Assessment are a matter of policy interest. Some of the declines may have resulted from shifts in the age-grade distributions of students in the Programme for International Student Assessment samples. We use multiple regression methods to model the student-level effects of grade for each Programme for International Student Assessment cycle allowing for the effects of student characteristics (e.g. socioeconomic background and gender) and jurisdiction. We estimate an average net effect of grade over the Programme for International Student Assessment cycles since 2006 as 42 scale points with no difference between reading and mathematics. We explore the extent to which differences between grades in achievement and changes in the grade distributions of students contributed to changes in average Programme for International Student Assessment achievement scores. We conclude that the relatively greater decline in Grade 11, compared to Grade 10 achievement, contributed to the overall decline and that shifts in distributions may have also contributed a little to those declines.