Countries differ in their upper secondary school systems in a way that some require their students to choose a specialization from a set of areas – typically natural sciences, economic sciences, humanities or arts – and follow that specialization for the course of their upper secondary education years (e.g. Portugal, Spain, Sweden) whereas by contrast, others including Finland, Denmark or the U.S. follow a general curriculum where students, albeit being able to choose between different classes in distinct areas, are not required to follow a single specialization and thus, receive a more general education. Because countries only follow one system or the other, a cross-country analysis is required to estimate the possible effects of these institutional differences. An international differences-in-differences approach is chosen to account for country heterogeneity and unobserved factors influencing student outcomes, by using both PISA and PIAAC data for 20 different countries. The regression results suggest that the choice of one system or the other does not account for differences across countries in either the mean performance or the inequality of students’ test scores.