The subjects that young people study from age 14 onwards may have important consequences for their future academic and labour market outcomes. These decisions are shaped by the schools in which they find themselves. Schools also face constraints of their own. This paper explores the extent to which individuals' decisions are affected by the school they attend and to what extent this is affected by the composition of schools in terms of academic attainment, gender, and socioeconomic background. We use multi-level variance decomposition models applied to administrative data on the subjects that young people in mainstream state-funded schools in England study between ages 14 and 16. Our results highlight the important role that constraints on schools play in subject choice decisions. We also note the particular role of attending a non-selective school within a selective schooling district.