Two hundred and nine pupils were randomly allocated to either a cognitive behaviourally based stress management intervention (SMI) group, or a non-intervention control group. Mood and motivation measures were administered pre and post intervention. Standardized examinations were taken 8–10 weeks later. As hypothesized, results indicated that an increase in the functionality of pupils’ cognitions served as the mechanism by which mental health improved in the SMI group. In contrast, the control group demonstrated no such improvements. Also, as predicted, an increase in motivation accounted for the SMI group's significantly better performance on the standardized, academic assessments that comprise the United Kingdom's General Certificate of Secondary Education. Indeed, the magnitude of this enhanced performance was, on average, one-letter grade. Discussion focuses on the theoretical and practical implications of these findings.