It is widely recognized that we need to know more about effects of class size on classroom interactions and pupil behavior. This paper extends research by comparing effects on pupil classroom engagement and teacher-pupil interaction, and examining if effects vary by pupil attainment level and between primary and secondary schools. Systematic observations were carried out on 686 pupils in 49 schools. Multilevel regression methods were used to examine relationships between class size and observation measures, controlling for potentially confounding factors like pupil attainment. At primary and secondary levels smaller classes led to pupils receiving more individual attention from teachers, and having more active interactions with them. Classroom engagement decreased in larger classes, but, contrary to expectation, this was particularly marked for lower attaining pupils at secondary level. Low attaining pupils can therefore benefit from smaller classes at secondary level in terms of more individual attention and facilitating engagement in learning. (Contains 4 tables and 7 figures.)