Aim: To investigate whether increased physical exercise during the school day influenced subsequent cognitive performance in the classroom. Method: A randomized, crossover-design trial (two weeks in duration) was conducted in six mainstream primary schools (1224 children aged 8-11y). No data on sex was available. Children received a teacher-directed, classroom-based programme of physical exercise, delivered approximately 30 minutes after lunch for 15 minutes during one week and no exercise programme during the other (order counterbalanced across participants). At the end of each school day, they completed one of five psychometric tests (paced serial addition, size ordering, listening span, digit-span backwards, and digit-symbol encoding), so that each test was delivered once after exercise and once after no exercise. Results: General linear modelling analysis demonstrated a significant interaction between intervention and counterbalance group (p less than 0.001), showing that exercise benefited cognitive performance. Post-hoc analysis revealed that benefits occurred in participants who received the exercise intervention in the second but not the first week of the experiment and were also moderated by type of test and age group. Interpretation: Physical exercise benefits cognitive performance within the classroom. The degree of benefit depends on the context of testing and participants' characteristics. This has implications for the role that is attributed to physical exercise within the school curriculum. (Contains 2 tables and 2 figures.)