I study how a significant increase in the compulsory schooling leaving age, from 15 to 18 years old, can contribute to reducing early school leaving and changing students’ educational paths. I analyse the Portuguese reform of 2009, exploiting the fact that grade retention in the 7th grade in this year provides quasi-experimental variation in exposure to the new policy. While effects for the overall student population are small or null, lower-achieving students significantly increase their schooling duration. Additionally, some sub-groups of lower-achieving students, particularly boys and those enrolling in upper-secondary school, increased their graduation probabilities. At the same time, I do not find that school quality decreased. These findings carry implications for research using compulsory schooling reforms as instruments for education, and inform policies aimed at supporting at-risk students.