This longitudinal study examines the long-term effects on the achievement of students at a diverse suburban high school after all students were given accelerated mathematics in a detracked middle school as well as ninth-grade ‘high-track’ curriculum in all subjects in heterogeneously grouped classes. This study used a quasi-experimental cohort design to compare pre- and post-reform success in the earning of the New York State Regents diploma and the diploma of the International Baccalaureate. Using binary logistic regression analysis, the authors found that there was a statistically significant post-reform increase in the probability of students earning these standards-based diplomas. Being a member of a detracked cohort was associated with an increase of roughly 70% in the odds of IB diploma attainment and a much greater increase in the odds of Regents diploma attainment – ranging from a three-fold increase for White or Asian students, to a five-fold increase for African American or Latino students who were eligible to receive free or reduced-price lunch, to a 26-fold increase for African American or Latino students not eligible for free or reduced-price lunch. Further, even as the enrollment in International Baccalaureate classes increased, average scores remained high.