It has been claimed that physical activity has a positive effect on not only health but also on school performance. Using data from a randomised school-intervention study, this paper investigates whether different interventions promoting physical activity affect school performance in primary school children. The results indicate that on average, the interventions have a very limited beneficial impact on the pupils' performance. The effects are mostly insignificant, and in some cases even negative. These results are relevant when considering how general school resources and resources targeted towards physical activity in particular should be invested to improve school performance.