Drawing on six years of historical research, this article presents a theoretical representation of the social mechanisms at work in the 30-year history of an ethnically diverse, public, magnet arts high school in an urban U.S. school district. Students' sense of belonging has been a significant factor in creation and maintenance of social capital in this school from its founding in 1971 through spring 2002. Merging the psychological concept of belonging with the sociological construct of social capital provides a functional understanding of social capital at work in a contemporary high school. Earlier scholarly work on both belonging and social capital in schools has failed to consider the possible agency of students as portrayed in this article. Students' sense of belonging emerges as a necessary addition to the theory of social capital in the evaluation of high schools' social functionality and a key to better student experiences in these institutions.