Selective and comprehensive school systems vary in both the degree and timing of selection. To study the consequences of such variation, cross-national comparisons are usually undertaken. Given that cultural differences between countries affect pathways and outcomes, apportioning causal influence in such studies can be difficult. In 1970s Britain, selective and comprehensive systems coexisted. This enables us to compare the influences of organisational arrangements without the complication of national cultural differences. We analyse, for children of various abilities, while taking account of gender and class, the effect on achievement of experiencing comprehensive or selective schooling. Assuming that contextual and individual factors work conjuncturally in producing outcomes, we employ Ragin's configurational Qualitative Comparative Analysis. By treating cases in the National Child Development Study as configurations of factors, we are able to analyse the sufficient and necessary conditions for achievement. We find that system differences affect only some high-ability children's educational outcomes. (Contains 9 notes, 3 figures and 11 tables.)