© 2016 by the Comparative and International Education Society. All rights reserved. This article analyzes whether school social segregation, derived from policies and practices of both between-school student allocation and within-school streaming, is related to the effectiveness of the Italian education system. Hierarchical regression models are used to set out territorially aggregated factors of social sorting influencing learning opportunities beyond the traditional north-south economic divide. The findings show that practices that foster or consent to uneven distribution of students between classrooms are likely to adversely affect the overall level of educational effectiveness, especially in those areas with the lowest levels of socioeconomic development. As regards the uneven between-school allocation, the findings show that what matterswhen it comes to assessing the negative effect of between-school segregation on educational outcomes is not the region of residence but whether pupils live in a metropolitan area. The results are discussed in light of the students’ heterogeneity management models found in the international arena.