Drawing upon data gathered from 9,301 Year 7 students (12-13-year olds) from 46 secondary schools in England, this study represents the first larger-scale attempt to compare their actual set allocations in maths with the counterfactual position where their allocation to sets is based solely on their prior attainment at the end of primary school (using their Key Stage 2 fine grained scores in maths). Through such an analysis, the study found that nearly a third of students (31.2%) had been misallocated to lower or higher sets than their Key Stage 2 results would have warranted. Beyond this, school setting practices were found to exacerbate differences in set allocation in relation to gender and ethnicity, but not socioeconomic background. The odds of girls being misallocated to lower sets in maths than their prior attainment would warrant found to be 1.5 times higher than boys. Similarly, the odds of Black students being misallocated to lower sets being 2.4 times higher than for White students, whilst the odds of Asian students being misallocated to lower maths sets was 1.7 times higher than for White Students. The article concludes by reflecting upon the significant role that setting by attainment in secondary school can play in exacerbating already established patterns of educational inequalities in gender and ethnicity.