Socioeconomic health differences (SEHD) are relatively small in childhood. In adolescence they almost seem to disappear and among young adults they re-emerge. This article deals with mechanisms that contribute to the emergence of health differences by studying a group of 10-11 year old children in The Netherlands (n = 908). The role of determinants of health in the relation between socioeconomic status and health (causation) is studied, as well as the influence of health on school performance (selection). Both causation and selection mechanisms prove to exist. Life style and life circumstances are unequally distributed among the socioeconomic groups and can (partly) explain the relation between socioeconomic status and health. The health of the children is related with school performance, which can be seen as health selection. This relation however was only found in the lowest socioeconomic groups. In the lowest socioeconomic groups less healthy children perform worse at school than healthy children. The unequal distribution of determinants of health and health selection in the educational career among children probably contribute to SEHD in adult life.