The purpose of this study was to examine how the members of children's peer groups resemble each other in terms of behavioural and emotional strengths, academic achievement and behaviour at school. The participants were 739 9- to 10-year-old children (354 boys, 385 girls) from 30 Finnish elementary schools. 431 children (241 girls, 190 boys) were part of peer groups. Less than 5% of participants were other than Finnish origin. The children self-assessed their behavioural and emotional strengths, and teachers evaluated the children's academic achievement and behaviour at school. Children also filled the sociometric questionnaire and sociograms were drawn to identify peer groups. Standardized beta values of regression analyses were used as indicators of effect sizes. The results showed that when observed as a strength, children's school functioning was positively linked to both academic achievement and behaviour at school. The intraclass correlations (ICC) revealed that members of peer groups resembled each other in terms of behavioural strengths, and behaviour at school, but not in academic achievement. The gender composition of peer groups explained the variance in behaviour at school between peer groups. The results of this study highlight the importance to consider children's peer group also when implementing behavioural interventions.