Two field studies of high school students' sense of obligation to nonclass school activities are reported. "Sense of obligation" is conceived of as mediating some relations between the student and the school environment and is defined as a personal feeling of "I ought to..." or "I must...," i.e., a personal constraint, with reference to attending and helping with group activities. In 1961, (N=40) and 1965 (N=80), samples of marginal (poorly suited) and regular (better suited) students in small and large schools were interviewed concerning "reasons for or pulls toward" attending selected nonclass activities, and responses were coded into categories indicating sense of obligation. Marginal students in small schools reported as much sense of obligation as regular students, while marginal students in large schools reported little, if any. Theoretical and metholological implications are discussed. Tables and charts summarize the data. (NH)