In an era when evidence-informed decision-making is mooted as a means of achieving equity, data are now being used to revive the in-school stratification of students, despite sustained concerns around the long-term negative consequences of these practices. This institutional ethnography explores how teachers use data in their everyday work, and how evidence-informed decision-making supports the logic of grouping students by ability. Research data are drawn from two Australian schools where achievement data were used to group students, in response to requirements to use evidence-informed practices. This had implications for enacted curriculum and pedagogies, as teachers used grouping to differentiate instruction. Although there were no formal directives to group by ability, a cascade of performance management policies was implicated in the rise of these practices. This use of data was normalised, and ability grouping practices were evident from the early years of schooling onwards.