This study uses longitudinal administrative data from two states covering 2005-2006 to 2011-2012 to examine the effects of exposure to International Baccalaureate’s Primary Years Program (PYP) on students’ academic performance in grades 3 and 5. I exploit within-school variation over time in the adoption of the PYP to estimate achievement impacts. Across both states, I find that exposure to the PYP improves the reading performance of economically disadvantaged third-graders (by about 0.10 standard deviations), with little evidence of any offsetting, harmful effects on the reading achievement of other students. Yet, I arrive at discordant conclusions about the effects of the PYP on students’ mathematics performance: In Michigan, estimates suggest that fifth-graders exposed to the PYP perform about the same as their non-PYP counterparts, and third-graders perform about 0.07 standard deviations better. In North Carolina, I find negative effects of the PYP on third-and fifth-grade math performance, especially among boys. I find no evidence that these negative impacts are mediated through differential changes in teacher turnover. I close with a discussion the practical significance of the main findings and suggest key areas of focus for future research.