This study investigates the impact of child labour on educational attainment over a three-year horizon. Whilst this question has been explored by a plethora of studies in the literature, research focusing on the impact on school performance in developing economies is scarce. Employing a newly available dataset from the Young Lives survey and an instrumental variables strategy, this study examines the impact of working at age 12 on mathematics test scores three years later for children in Vietnam. In rural areas the evidence is suggestive that the impact of child labour is negligible. In urban areas, however, there is causal evidence that child labour significantly impedes educational attainment; a one standard deviation increase in hours worked reduces mathematics test scores by 12.45 points out of 100, or 67.85% of one standard deviation of the test score.