Prior achievement is essential to estimating the role of schools and school factors on student outcomes because it measures students' pre-existing knowledge and skills. However, its very strong effects and their implications for research and policy are not widely appreciated. Analyses of student achievement in five domains shows that prior achievement, measured 2 years before, has overwhelming effects, albeit with domain and year-level (grade) differences. When considering prior achievement, only a small minority of schools have effects that significantly differ from the average school effect on student performance. The variation in school effects is quite limited, and there are only trivial jurisdictional differences in school effects. The contemporaneous effects of parents' occupational group and education -- factors prominent in school funding in Australia -- are negligible. These findings are likely to pertain to other educational contexts since prior achievement typically has strong, or very strong, relationships with achievement.