This paper addresses how features of educational systems influence the association between social background and educational disadvantage up to the end of secondary school. Boudon's ideas about the primary effects are brought together with preschool participation and the secondary effects are argued to be strongly related to the stratification of the educational system. The analyses are based on around 35,000 respondents from 29 countries provided by PIAAC. Two-step estimations combining logistic regression models within educational systems (first step) with estimated dependent variable models between educational systems (second step) are applied. The results suggest that a higher preschool participation rate is associated with a lower dependency of educational deprivation (low achievement) on social background in the early years after finishing secondary school.