Background: Given the strong relationship between executive functions and academic achievement, there has been great interest in improving executive functions. School-based group interventions targeting executive functions revealed encouraging results in preschoolers and young school children; however, there is a paucity of studies in older primary school children (age 10-12). This is surprising considering that deficits in executive function performance can often be observed in this age group. Aims: Therefore, the aim of this study was to examine whether an innovative customized school-based group intervention could improve core executive functions (updating, inhibition, and shifting) in the age group concerned. Sample: In total, 118 ten- to twelve-year-old school children were recruited from eight participating classes. Methods: They were randomly assigned to one of two-six-week conditions of either a "cognitive games group" comprising of card and board games training executive functions (experimental group) or a "wait-list control group" (regular school lessons). In the cognitive games group, the class teachers held a 30-min training session twice a week. ANCOVAs (using pre-test values as covariate) were used to compare executive function performance between groups. Results: Results revealed that the cognitive games group improved specific executive functions (updating and shifting) compared to the control group. These findings indicate that a school-based group intervention can improve executive functions, even in 'older' primary school children. Conclusion: This study provides empirical evidence for the effectiveness of a classroom-based cognitive training in older primary school children and is of practical relevance for educators.