Using data collected by the Birth-to-Twenty child cohort study in urban South Africa, this paper describes the patterns of schooling of a population of children born in the Greater Johannesburg area in April to June 1990. This paper examines the patterns of initial enrolment in Grade 1, transitions through grades, and trends in primary school completion. The vast majority of children in the Birth-to-Twenty cohort entered school in the year they turned 6 or 7, and almost all were still enrolled at the age of 15, the end of compulsory schooling in South Africa. While only a very small number of children in the study are excluded from formal schooling, the study reveals relatively high rates of repetition, specifically for boys living in working-class and poor neighbourhoods. (Contains 5 tables, 3 figures, and 4 notes.)