Countries differ in their academic upper-secondary school systems whereby some require their students to choose a specialization track from a set of areas – typically natural sciences, economic sciences, or humanities – and follow that specialization for the course of their upper secondary education years whereas, by contrast, others follow a general curriculum in which students are not required to follow a single specialization and thus, may receive a more general education. Because countries follow only one system or the other, a cross-country analysis is required to estimate the possible effects on students’ achievement of these institutional differences. We measure achievement with the scores in reading and mathematics in international assessment programmes. An international difference-in-differences approach is chosen to account for country heterogeneity and unobserved factors influencing student outcomes, by using data from three different large-scale international student assessments. Results suggest that there is a negative effect from specialization tracking and that this may be greater for specific sub-groups of the student population – in particular, for students with a low socioeconomic status.