Access to higher education is affected by inequalities worldwide. Here we present a longitudinal study based on large databases of scores in upper secondary education and access to higher education in Portugal. Our findings show how access to higher education builds on and reinforces social inequalities: (1) private, fee-paying secondary schools inflate their students' scores; (2) this inflation unfairly improves their chances of accessing higher education; (3) the Portuguese national compensatory education programme is ineffective in bridging the gap in academic performance between the less privileged and the rest of the schools. This study, then, provides further insights into how the upper classes are disproportionately benefited by the massification of higher education. To achieve greater equity, issues such as the State supervision of grading practices, the assessment and reconfiguration of compensatory education programmes, and the admission process into higher education have to be addressed.