In this study, I examine whether eleventh-grade students' enrollment in the Chicago Public Schools (CPS) International Baccalaureate (IB) Diploma Program improves their academic achievement as measured by their ACT examination scores, probability of high-school graduation and probability of college enrollment. CPS offers the IB Diploma Program in thirteen high schools, more than twice any other U.S. school district. Using data on the IB enrollment status of 20,422 students attending these thirteen high schools from 2002-2008, I estimate that IB enrollment increases students' academic achievement by as much as 0.5 standard deviations and their probability of high-school graduation and college enrollment by as much as 17 and 22 percentage points respectively. All of my estimates are highly robust to validity threats posed by self-selection into IB enrollment. All estimates are greater for boys than for girls. I also calculate that the IB Diploma Program is a cost-effective way to increase high-school graduation rates.