Background Given the well-known relation between intelligence and school achievement we expect children with normal intelligence to perform well at school and those with intelligence deficits to meet learning problems. But, contrary to these expectations, some children do not perform according to these predictions: children with normal intelligence but sub-average school achievement and children with lower intelligence but average success at school. Yet, it is an open question how the unexpected failure or success can be explained. Aims This study examined the role of working memory sensu Baddeley (1986) for school achievement, especially for unexpected failure or success. Method and procedures An extensive working memory battery with a total of 14 tasks for the phonological loop, the visual-spatial sketchpad and central executive skills was presented in individual sessions to four groups of children differing in IQ (normal vs. low) and school success (good vs. poor). Outcomes and results Results reveal that children with sub-average school achievement showed deficits in working memory functioning, irrespective of intelligence. By contrast, children with regular school achievement did not show deficits in working memory, again irrespective of intelligence. Conclusions and implications Therefore working memory should be considered an important predictor of academic success that can lead both to unexpected overachievement and failure at school. Individual working memory competencies should be taken into account with regard to diagnosis and intervention for children with learning problems.