Preparing K-12 students for careers in science, technology, engineering and mathematics (STEM) fields is an ongoing challenge confronting state policymakers. We examine the implementation of a science graduation testing requirement for high-school students in Massachusetts, beginning with the graduating class of 2010. We find that the design of the new requirement was quite complicated, reflecting the state’s previous experiences with test-based accountability, a broad consensus on policy goals among key stakeholders, and the desire to afford flexibility to local schools and districts. The consequences for both students and schools, while largely consistent with the goals of increasing students’ skills and interest in STEM fields, were in many cases unexpected. We find large differences by demographic subgroup in the probabilities of passing the first science exam and of succeeding on retest, even when conditioning on previous test-score performance. Our results also show impacts of science exit-exam performance for students scoring near the passing threshold, particularly on the high-school graduation rates of females and on college outcomes for higher-income students. These findings demonstrate the importance of equity considerations in designing and evaluating ambitious new policy initiatives.