High school graduation rates are a central policy topic in the United States and have been shown to be stagnant for the past three decades. Using student-level administrative data from New York City Public Schools, I examine the impact of compulsory school attendance on high school graduation rates and grade attainment, focusing the analysis on ninth and tenth grade cohorts. I exploit the interaction between the school start-age cutoff and compulsory attendance age requirement to identify the effect of compulsory schooling. I find that an additional year in compulsory attendance leads to an increase of 9 to 12 percent in the probability of progressing to grades 11 and 12, and raises the probability of graduating from high school by 9 to 14 percent, depending on the specification.