No previous study on the interrelation between parental birth order and offspring educational attainment has analysed the first stage of tracking into schools, which in many countries is highly selective of persons who aim for university studies. We do so and examine how parental birth order is associated with attainment of general upper secondary education in Finland. Three-generational register data on the total population are used to assess how the probability of having the matriculation examination at age 20 is associated with parental birth order. The total number of individuals in the child generation is 377,038. We find that the probability of having the matriculation examination decreases with parental birth order. Within-family analyses using cousin fixed effects models, which account for unobserved characteristics in the extended family, show that having later-born parents, and in particular a later-born father, is negatively associated with first tracking into schools. These patterns can solely be attributed to the fact that earlier-born parents are highly educated and found in higher social classes than later-born parents. The results largely corroborate findings from similar analyses of length of schooling in Sweden, although the contribution of parental education and social class is notably stronger in our study context.