Research in clinical samples suggests that the relationship between intelligence and academic achievement might be moderated by sustained attention. The present study aimed to explore whether this interaction could be observed in a non-clinical sample. We investigated a sample of 11th and 12th grade students (N = 231). An overall performance score and a quality of performance score in sustained attention as well as verbal, numerical, and general intelligence scores served as predictors. Achievement criteria were grades in math and German as well as Grade Point Average (GPA) obtained after testing. Both types of sustained attention scores were significantly related to school performance, but only the quality of performance score incrementally contributed to the prediction of school performance above and beyond intelligence. Overall differences in the sustained attention test significantly moderated the relationship between verbal intelligence and German grades. Quality of performance moderated the relationship between general intelligence and GPA as well as the one between numerical intelligence and math grades. The study elucidates the interplay between sustained attention, intelligence, and school performance in a non-clinical sample. (Contains 3 figures and 3 tables.)