For many years studies of educational inequality have been widely spread in social sciences. Since popular Coleman report attention is drawn to the achievement gaps between students from different social classes. The socio-economic status of the individual refers to the amount of social and cultural capital. In most studies it demonstrates positive connection to academic achievement. Diverse short-term and long-term results of education for students with varied socio-economic characteristics produce an acute issue for researchers and policymakers. On the school level the composition of students’ body is considered as the most influential characteristic for educational results. Particularly researchers have found a positive compositional effect of socio-economic status. The compositional effect implies a value-added impact of students’ socio-economic status defined at the school level on their individual academic performance. There are some theories to explain the presence of compositional effect in general. The impact of school composition can be based on peers’ characteristics, school processes and teachers’ work. All these aspects taken together can potentially shape the connection between school socio-economic composition and students’ achievement during education and after finishing school. Therefore, it is supposed that classmates’, school’s and teachers’ characteristics may serve as mediating mechanisms for establishing the connection between school composition and students’ performance. At the present time, the phenomenon of the socio-economic composition effect is not investigated thoroughly. The existence of universal connection between school socio-economic composition and students’ performance for various cultural contexts is not clear enough. While some authors provide empirical evidence to support the presence of the effect, others demonstrate the opposite results. Besides, it is claimed that several flaws in methodology can lead to discovering significant effect of school composition as a statistical artefact. At the same time, previous studies on this topic explored mostly the effect of school composition itself while mediating factors on which this connection is based were neglected. It is not discovered yet, how the compositional effect works when it is found, and which aspects serve as mediators of this connection. From the point of the development of educational reforms aimed at reducing the negative consequences of social inequality, it is important to understand exactly which aspects of the educational process play more significant role in establishing the compositional effect. Thus, the results of the analysis of the mechanisms of the connection between socio-economic composition of the school and educational results can serve as the basis for the formation of evidence-based recommendations for the development of policy measures that help to solve the problem of social inequality. In the present study, a comprehensive analysis of the mechanisms of the functioning of the link between school socio-economic composition on the one hand, short-term and long-term educational results, on the other is implemented. The purpose of the work is to determine whether there is a link between socio-economic composition and educational results, and to describe the mediating mechanisms. 

In comparison to previous works the design of the research along with the longitudinal data provides opportunities to follow the recommendations for compositional effects analysis (e.g. prior achievements inclusion) and compare the mechanisms of the connection between short-term and long-term results. Secondary data from the Russian longitudinal study "Trajectories in Education and Occupations" (TREC) (https://trec.hse.ru) were used for the research. The database provides information on short-term and long-term educational achievement as well as a variety of school and students’ characteristics which could be used as measures of mediating mechanisms. The TREC survey was focused on the trajectories of Russian students in the transition from school to the labor market. The longitudinal survey started in 2011, and the last wave at the moment was collected in 2017. In general, the survey consists of six waves and includes data from international surveys of the quality of education “TIMSS” 2011 (timssandpirls.bc.edu) and “PISA” 2012 (www.oecd.org/pisa/). To analyze the data multilevel regressions and Structural Equation Modeling (SEM) were used. In the current study we used SEM with cluster correction to calculate robust standard errors. To check for mediation and indirect effects the traditional three-step framework was applied.

We discovered that there is a strong positive effect of school socio-economic composition on short-term performance and long-term academic trajectories. At the same time, it was discovered that besides direct effect of composition there is an indirect effect on short-term performance through teacher’s expectations about the progress and classmates’ academic aspirations - on long-term. Thus, the mechanisms of the connection between school socio-economic composition and students’ academic performance are different for short-term and long-term achievement as it was hypothesized previously in the literature. At the same time, the results suggest that none of the school characteristics that were investigated could serve as a mechanism of social reproduction. It implies that investments in school recourses, revision of the curriculum or promotion of healthy environment developing academic goals may not bring crucial changes to the reduction of social inequality. On the other side, work with teachers and promotion of higher educational programs for students could potentially be methods for reducing inequality.

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