Recent research has increasingly been studying the long-lasting effects of secondary education structures and processes on higher education (HE) outcomes. While the influence of socioeconomic composition on higher education enrolment is established, the underlying mechanisms remain unclear. We posit that the composition effect partially runs through the educational expectations of the students' peers at school. By considering shared post-secondary expectations among students within a school -- or, peer expectation culture -- we transcend the dominant individual approach to expectations. Additionally, we go beyond the supposed positive (i.e. normative) effects of peers on HE enrolment, by considering the negative effects of having ambitious peers on HE enrolment resulting from comparative group processes. Logistic multilevel analyses of longitudinal data (2013-2019) from 1250 students across 30 Flemish high schools showed that socio-economic status (SES) composition effects on HE outcomes were mediated by expectation culture. Students attending schools with high expectation cultures were more likely to attend HE in general, and to attend university over other HE institutes compared to students in schools with low expectation cultures. The results suggest normative effects of peers on both outcomes, while comparative processes were only found for the choice of institution. This study underscores the long-term consequences of both individual and school-level expectations in secondary education and calls for including comparative reference group effects in studies on educational decision-making.