This paper examines the causal impacts of Hong Kong's 1971 policy of free compulsory education on students' educational attainment. Using a regression discontinuity method and Hong Kong Census data, this study compares children born just before and just after the month in which the compulsory-education law came into effect. The results show that the law reduces approximately 10 percent of the dropout probability by age 12 and 8 percent of the dropout probability by age 15. The effect is substantial considering that approximately 90 percent of primary-school-age children were already in school in Hong Kong when the policy was implemented. The policy has larger impacts for socially and economically disadvantaged children, and the law also increases their probability of obtaining an education beyond middle school. Robustness is tested with several sensitivity checks. The results will help policy makers and stakeholders better understand the potential efficacy of mandated education policies and increasing educational access to targeted populations, especially for developing economies.