Research has consistently shown ‘ability’ grouping (tracking) to be prey to poor practice, and to perpetuate inequity. A feature of these problems is inequitable and inaccurate practice in allocation to groups or ‘tracks’. Yet little research has examined whether such practices might be improved. Here, we examine survey and interview findings from a large-scale intervention study of grouping practices in 126 English secondary schools. We find that when schools are encouraged to allocate students and move them between groups according to (...) equitable principles by participation in a ‘best practice’ intervention, there is some increased equity of practice (i.e. a reduction in non-attainment factors used in allocation). However, the majority of schools continue to use subjective and potentially biased information to group students. Furthermore, some schools that claim to be using attainment setting appear to be using the inequitable practice of streaming. Our findings show that improvements in equity are constrained by operational and strategic factors, including timetabling, finance, and teachers’ values and beliefs relating to student ability and progression. We suggest strategies for encouraging schools to change their grouping practices, drawing on approaches for working with complex organisations. (shrink)