We explore how school experiences and social networks structure young people's substance use in different institutional contexts. The concepts of 'selection' and 'influence' are situated within the context of bounded agency, counter-school cultures and Bourdieusian notions of capital. We employed individual and group interviews, network-mapping, and observations at two contrasting English secondary schools. Both schools were characterised by extended social network structures that appeared to influence patterns of substance use, although the mechanisms via which this occurred varied according to school context. At Grange House school (suburban context) a minority of students from disadvantaged families were alienated by the attainment-focused regime, marginalised by a strong peer-led centrifugal force pushing them outwards, and substance use was an alternative source of bonding and identity for these students. In contrast, at North Street a centripetal force operated whereby the majority of students were pulled towards highly-visible, normative markers of 'safe', 'road culture', such as cannabis use and gang-involvement, as they attempted to fit in and survive in an inner-city school environment. We conclude that health inequalities may be reproduced through these distinctive centrifugal and centripetal forces in different institutional contexts, and this should be the focus of quantitative examination in the UK and elsewhere.