High and stable levels of aggression and the presence of aggressive behaviour in multiple settings according to different informants are risk factors for later problems. However, these two factors have not been investigated in early childhood. The present study investigates trajectories of parent-reported child aggression from 1.5 up to 6 years of age and their association with aggressive behaviour, attention problems and rule-breaking behaviour in a different setting, as reported by the teacher. In a longitudinal population-based cohort study, parent-reported measures of aggressive behaviour were obtained using the CBCL when children were 1.5, 3, and 6 years of age (n = 4,781). Teacher-reported problem behaviour at school was assessed at age 6.5, using the TRF questionnaire (n = 2,756). Growth mixture modelling yielded three aggression trajectories, with high increasing (3.0%), intermediate (21.3%), and low decreasing (75.7%) aggression levels. Children in trajectories with higher and increasing levels of aggression showed more teacher-reported aggressive behaviour, attention problems and rule breaking behaviour. However, parent-reported aggression at age six predicted problem behaviour at school to the same extent as did the aggression trajectories, suggesting that the incremental value of trajectories is not always self-evident.