Background: We explored whether school-based Cogmed Working Memory Training (CWMT) may optimise both academic and psychological outcomes at school. Training of executive control skills may form a novel approach to enhancing processes that predict academic achievement, such as task-related attention, and thereby academic performance, but also has the potential to improve the regulation of emotion, social problems and behavioural difficulties. Methods: Primary school children (Mean age = 12 years, N = 148) were cluster-randomised to complete active CWMT, a nonadaptive/placebo version of CWMT, or no training. Results: No evidence was found for training effects on task-related attention when performing academic tasks, or performance on reading comprehension and mathematics tasks, or teacher-reported social, emotional and behavioural difficulties. Conclusions: CWMT did not improve control of attention in the classroom, or regulation of social, emotional and behavioural difficulties.