The existence of a contextual effect of the social class composition of a school's intake on individual student performance--the school mix effect--has long been debated in qualitative school effectiveness literature. This paper reports on a qualitative study of four New Zealand urban secondary schools of varying social class composition (and ethnicity) designed to shed light on a school mix effect by examining possible causal mechanisms. One low-socioeconomic status school was compared with three middle-class schools. Evidence was found to support a whole-school explanation for a school mix effect stemming from the cumulative effects of reference groups, instructional practices, and organizational and management practices. Implications for research and policy are suggested. Addressing the effects of school mix effect should be part of the policy agenda for reducing educational inequity. (Contains 1 table, 2 figures, and 60 references.) (Author/SLD)