Aim: We investigated the dose--response relationship between acute physical activity (PA) intensity during physical education (PE) lessons (dose), and task behaviour and learning experiences in the classroom after PE (response), and mediation effects of acute PA on-task behaviour via learning experiences. Method: A total of 78 children (M[subscript age] = 9.30 years; 43 females) took part. Participants reported learning experiences (tiredness, positive and negative affect) during one afternoon per week for 6 weeks. Their task behaviour was observed (on-task, active off-task and passive off-task) during two classroom lessons. Between the classroom lessons, they took part in a PE lesson, with experimentally induced PA intensity (low, medium and high). Accelerometers were worn for 24 h leading up to and during every intervention afternoon. Participants completed self-reports three times per classroom lesson, both before and after PE. Intra- and interindividual differences in PA, task behaviour and learning experiences were analysed with multilevel structural equation models. Results: Moderate PA directly increased on-task behaviour and reduced passive off-task behaviour, whereas light PA increased active off-task behaviour and reduced on-task behaviour. We found no direct effects of vigorous PA or mediated effects of any PA intensity on-task-related behaviour. However, a greater positive affect during PE indirectly led to more on-task and less passive off-task behaviour. Regularly active children reported less tiredness in the classroom. Conclusion: PE lessons can increase on-task behaviour and reduce both passive and active off-task behaviours. Positive affect and tiredness are indirectly involved in the impact of PA on task-related behaviour. The greatest benefits were found for moderate PA and for PE lessons, which left children feeling positive. Moreover, regular participation in moderate-to-vigorous PA leads children to feel less tired during school lessons.