Young people in Irish schools are required to choose whether to sit lower and upper secondary exam subjects at higher or ordinary level. This paper draws on a mixed methods longitudinal study of students in twelve case-study schools to trace the school and student factors influencing take-up of higher level subjects within lower secondary education. School organisation and process are found to shape the extent to which young people actually have a "choice" or whether this is circumscribed by the school they attend or the class group to which they are allocated. Streaming practices, which are more prevalent in schools serving socioeconomically disadvantaged communities, constrain the degree of choice young people have over their subject levels, with those in lower stream classes usually allocated to ordinary level. Even where schools have mixed ability base classes, schools influence access to higher level subjects. In the middle-class and socially mixed schools, teachers are more likely to expect and encourage all students to take higher level, at least for as long as possible. In contrast, in working-class schools, there are sharp declines in the proportion taking higher level subjects as they approach the national exam taken at the end of lower secondary education. Early decisions about not pursuing higher level are found to have long-term consequences by closing off particular pathways for the future. These early decisions are often made in the absence of formal school-based guidance, thus contributing to social inequalities in young people's destinations. The findings contribute to our understanding of how curriculum differentiation reinforces social class differences in educational pathways.