Student engagement is widely viewed as an important antecedent to learning and achievement; however, research finds that engagement declines sharply as students advance through school. This article uses a mixed-methods approach to examine the engagement of high school seniors working on the IB Diploma Program's extended essay. Average engagement levels differ widely across the 8 school programs studied; however, school and programmatic features, including structure and support practices, do not explain these results. Qualitative analyses in two case study schools suggest that the culture of the cohort may be the most important contextual factor shaping students' engagement in the project. (Contains 5 tables, 1 figure, and 1 endnote.)