The issue of school segregation warrants serious policy concern as empirical evidence suggests that it can have significant effects on students’ educational opportunities. Indeed, a great many studies have examined the impact of school segregation (and desegregation) on students’ achievement levels and inequalities among them. Although many of these studies have been found to present a host of methodological problems (Schofield 1995), the argument that school segregation challenges student outcomes because of the existence of significant school composition effects is a well-established one. In other words, a rigorous assessment of the scope and functioning of the school composition effect is key when dealing with the issue of segregation and its possible impacts on educational efficiency and equality.

Within the research centered on school composition, particularly prolific are studies on the effects of the socioeconomic (SES) composition of schools and/or classes (see Van Ewijk and Sleegers 2010a, for a meta-analysis of studies on the effects of the socioeconomic status of peers on student achievement) and of the ethnic composition of schools or classes on student performance (see Van Ewijk and Sleegers 2010b, for a meta-analysis of peer ethnicity and achievement research). Of particular interest are those studies that include in their assessments covariates for both the socioeconomic and the ethnic dimensions of school composition (Rumberger and Palardy 2005; Van der Slik 2006; Dronkers and Levels 2007; among others). Moreover, given that they better tackle the problems of model specification and reliability of predictors (Harker and Tymms 2004) as well as the problem of selectivity bias, particular attention should be paid to those studies which, in addition to socioeconomic and ethnic composition, account for the effects of other composition variables (such as ability and gender) and include key variables of school processes in the models (namely, types of pedagogical practices and school management strategies) (De Fraine et al. 2003; Opdenakker and Van Damme 2001; Dumay and Dupriez 2007, 2008; Willms 2010; Hanushek et al. 2003).

Several studies have supported the differential sensitivity argument for school composition effect, in the sense that school composition matters most for the underprivileged (Sammons et al. 1993; Kahlenberg 2001; Robertson and Symons 2003; Zimmer and Toma 2000) and students of ethnic minorities (Hoxby 2000; Hanushek et al. 2002; Andersen and Thomsen 2011; Jencks and Mayer 1990; Hochschild and Scovronik 2003). Moreover, scholars have also found empirical support for an interaction effect between individual ability and school composition on achievement (De Fraine et al. 2003; Opdenakker and Van Damme, 2001; Thomas et al. 1997; Luyten and van der Hoeven-van Doornum 1995). Nonetheless, studies expanding on the repercussions that non-linear composition effects can have on learning outcomes in aggregated terms are scarce.

This article contributes to the existing literature on school segregation effects, taking a step further towards the identification of the methodological and policy implications of the non-linear nature of school composition effects, and exploring the contextual conditions under which the socioeconomic composition of schools affects educational efficiency and equality to a greater or lesser extent.

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