Introduction  

Educational equity is one of the main concerns in  both European countries and other major economies in the world,  which is also a hallmark of a successful  educational system.  Thus, main economies in the world aim to  reduce the relationship between students’ social economic status (SES) and school achievement, both at individual level and at school level.

Students’ SES  has been identified as one of the strongest individual attributes influencing student achievement in school (Sirin, 2005). Over and above the influence of individual students’ SES,  the  school  social composition or mean school SES also has  effects  upon student achievement. This is identified as school compositional effects or school mix effects. According to reports of the Program of International Student Assessment (PISA), in most of the OECD countries school compositional effects substantially outweigh individual SES effects upon student achievement (OECD, 2004; OECD, 2010a). Recent studies suggest that the effects of school composition are similar for low SES and high SES  students (Perry & McConney, 2010; OECD, 2004).

Harker and Tymms (2004)  summarized previous  researches and attributed  school compositional effects to three sources: peers, teaching and facilities. In high SES schools, students tend to appreciate academic achievement and experience favourable peer press to achieve academic excellence. From psychological perspective, high level  interactions among students in high SES schools also stimulate cognitive development. As a result, the achievements of all students will be lifted above the expectations  associated with  factors at individual level (Harker & Tymms, 2004).

PISA studies suggest that high-performing schools are also characterized with good teacher-student relationship,  teachers’ high expectations, high student morale and  disciplinary climate (OECD, 2010b). These are typical characteristics of high SES schools. Good teacher-student relationship and  high teacher expectations are of great importance for successful school learning, particularly for disadvantaged students (Jussim & Harber, 2005; Jennings & Greenberg, 2009). From instruction perspective,  high SES schools cherish amiable student-teacher relationship and  fewer interruptions in class (Rumberger & Palardy, 2005).  Meanwhile, the high level interactions between students and teachers in high SES schools may also motivate teachers to teach more challenging  topics in class.

Additionally, high SES schools are also well resourced,  allowing them to purchase sufficient  school facilities as well as hire competent and experienced teachers. All these factors are largely absent in  the majority of low SES schools.

There are established evidences that school social composition  affects school achievement through  school process: including  school climate, teachers’ expectation, teacher-student relationship, teachers’ and students’ morale, disciplinary climate etc. Nevertheless, there is little research on how far these school process factors mediate school composition effects. This paper is to study to what extent these school process factors mediate  compositional effects upon student achievement,  based on the data of 28 OECD countries  in PISA 2003.

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