As a way to raise attainment, schools are encouraged to form ability groups across classes for particular subjects. There is limited evidence however about the relationship between setting and primary school children's characteristics and how teachers perceive them. The aim of this study was to investigate associations between set positioning for English and Maths and teachers' perceptions about 11-year-olds' behaviour and school attitudes and future education goals, and children's psychosocial and cognitive characteristics and background through secondary data analyses of the fifth sweep of the Millennium Cohort Study. The study employed binary logistic regression analyses and revealed strong associations between set positioning and teacher perceptions of 11-year-olds' behaviour and attitudes and future education goals. Associations between set position and teacher perceptions of children's identity as learners were stronger than those for children's self- reported attitudes and behaviour. Also, family income and gender emerged as strong predictors of children's position in sets. Set positioning is not a neutral act; it has implications about children's personal agency and their identity as learners.