The extent to which school mix--the social class composition of schools--influences school processes and student achievement has long been a matter of dispute. Drawing on a comparative ethnography of four New Zealand urban secondary schools, this article suggests that school mix probably does impact on school organizational and management processes so as to drag down the academic effectiveness of schools in low socioeconomic settings and boost effectiveness in middle class settings. The four schools represent a range of socioeconomic status populations, with varying proportions of White, Maori, and Pacific Islander students. Evidence of the influence of school mix on daily routines, curriculum management, and guidance and discipline matters is discussed. The article provides a framework for explaining why school mix might influence school organization and management, and it suggests implications for educational markets, school self-management, and school evaluation. (Contains three tables.) (Author/SLD)