School markets and their influence on grades have been a topic of much discussion both in academia and beyond. Discussions regarding grade inflation has for example been a topic on documentary TV-shows as well as in different academic contexts. The related question of how school markets function in relation to the reinforcement of the influence of social background on school performance has however not been investigated at length. This paper aims to shed light on this question: "To what extent can the connection between social background and GPA’s in year 9 in compulsory school be explained by the emergence of school markets?". To help to answer this question data from the UGU-database from Gothenburg University was analysed using Structural Equation Modeling (SEM). Cohorts born in 1992, 1998 and 2004 was examined to be able to trace the influence of the expansion of school markets in Sweden over time. A mediation model was constructed utilizing a definition of school markets as the share of private school students' in a given municipality. The mediating function of this variable was measured in relation to the relationship between parental education background and the GPA for year 9-students in compulsory school. Gender and migration background were included as control variables. The resulting model was shown to have an acceptable model fit: l χ2 = 332,747, df = 6 (p = <.001). CFI = .955, NFI = .955. RMSEA = .043 (95 % C.I. .039-.047, p = .998). The main results include a 200.87 percent increase in mediation from the 1992-cohort to the 2004-cohort, suggesting that the emergence of school markets acts as a progressively stronger reinforcing agent in the relationship between social background and GPA's. This in turn suggests that the school market expansion plays a larger and larger role in enhancing the socially reproductive component of the school system.