Background/Context: There is broad agreement about the benefits of taking AP and/or IB courses in high school. Nonetheless, student access to such courses remains uneven and inequitable, due largely to the practice of tracking students by perceived "ability." These tracking practices are often defended based on the contention that detracking and mixed-ability classes are impractical or unworkable. This study's conclusions inform the policy debate on the efficacy of detracking as an instructional strategy and add to the emerging literature concerning the potential of providing a school's most challenging, highest quality curriculum to all students. Research Questions: We study a reform that combines two basic elements: detracking in Grades 6 through 10 plus open IB enrollment in Grades 11 and 12. We answer four questions: (a) Did greater numbers (and proportions) of students enroll in IB courses as the district progressively detracked its math and ELA courses? (b) As detracking took place over time, did IB courses become accessible to a broader range of students with respect to prior achievement? (c) How did students who enrolled in IB courses after detracking perform on their end-of-course IB assessments, conditional on prior achievement? Here we particularly focus on whether high-performing students appeared to perform worse on the IB assessments as the IB classes were composed of higher numbers of students with lower prior achievement. (d) Conditional on taking a math IB course, did students become less likely to take the more challenging Math SL IB course (relative to the less challenging Math Studies IB course)? Intervention/Program/Practice: Policy to detrack access to high school IB courses. Research Design: We use an interrupted time series approach to examine whether the onset of detracking coincided with (a) increased IB participation or (b) decreased IB scores. We also document whether low-, middle-, or high-prior skill groups of students perform less well during detracking. Finally, we explore whether racial achievement gaps on statewide assessments were exacerbated by detracking. Recommendations: The results associated with this detracking reform challenged two widespread beliefs. First, the school's highest achievers continued to succeed in the more heterogeneous IB classes. Second, the average IB scores for the school's lower achievers were the same or higher after detracking began, even though many more such students enrolled in those courses. In short, this case study documents the potential for not rationing the enriched, world-class curriculum of the International Baccalaureate.