This paper aims to analyze the effects of certain characteristics of the educational systems on the social composition of schools. After accounting for significant effects of schools' social composition on student outcomes (this is confirmed on the basis of a multilevel analysis), we explore the impacts of distinct components of what we name "school regimes" on measures of school social segregation (Hutchens indices) across countries and regions. The PISA 2006 database has been used as the main source of information for such measures. Our analysis considers data for 32 OECD educational systems. Certain characteristics of school regimes are specially assessed: the level of institutional differentiation existing in the educational career; the presence of private schools in compulsory education; the level of school autonomy as regards the process of student admission; and the models and criteria defining public regulation of school access processes. Results of the regression analyses suggest that more market-oriented school regimes tend to increase schools' social segregation, whilst those characterised as more comprehensive and publicly regulated tend to reduce it. (Contains 4 tables, 1 figure and 6 notes.)