Policies aiming to promote the mainstreaming of students with special educational needs in regular classrooms have become a focal point of political discussions in many countries. However, there is a scarcity of quantitative evidence with robust empirical designs that can shed light on the long-term educational outcomes associated with mainstream education. This study utilizes extensive longitudinal register data and employs a propensity score matching framework to investigate the disparities in upper secondary education completion between students educated in regular and segregated learning environments. The findings reveal that the likelihood of attaining an upper secondary certificate is significantly higher (by 18 percentage points) among students in regular classrooms. Furthermore, tentative results indicate that higher passing rates at the lower secondary school leaving exam may play a mediating role.