Worldwide, an increasing number of students seek private supplementary tutoring, known as 'shadow education.' Various studies report social class differences in the use of shadow education. High-SES families may invest in shadow education as a form of concerted cultivation, seeking to improve their children's school achievement. In this study, we apply meta-analytic structural equation modeling to explore relationships between parental education, income, and the use of shadow education across nations and educational contexts. We find robust relationships between parental education, income and the use of shadow education. Moreover, we assess a mediating role of shadow education in the relationship between SES and achievement. Shadow education appears to fulfill a competitive function for privileged families who seek to secure advantage in educational competition. We conclude that educational research, particularly research concerned with inequality of opportunities, needs to take account of the progressively prominent position of shadow education in the educational landscape.