The present study investigated the development of executive functions (EFs) and their associations with performance and behavior at school in 8-12-year-old children. The EFs were measured by computer-based n-back, Continuous Performance and Go/Nogo tasks. School performance was evaluated by Teacher Report Form (TRF) and behavior by TRF and Child Behavior Checklist. The studied dimensions of EF were cognitive efficiency/speed, working memory/attention and inhibitory control. Strong age effects were found for these cognitive abilities (p values less than 0.01). Inhibitory control was associated with better adaptive functioning (learning, working hard and behaving well), academic performance and less psychiatric symptoms (p values less than 0.05), specially in 8-9-year-old children. In this youngest age group low inhibitory control was also associated with teacher-reported inattention (p = 0.042). Low inhibitory control was associated with teacher- and parent-reported internalizing symptoms (p less than 0.01). These results suggest that maturational factors may underlie low adaptive functioning and psychiatric symptoms during early school years. Further studies are needed to evaluate the association between inhibition and emotional symptoms.