This study aimed to examine the impact of a universal, school-based intervention, the Good Behavior Game (GBG), on children's behavior, and to explore any subgroup moderator effects among children at varying levels of cumulative risk (CR) exposure. A 2-year cluster-randomized controlled trial was conducted comprising 77 primary schools in England. Teachers in intervention schools delivered the GBG, whereas their counterparts in control schools continued their usual provision. Behavior (specifically disruptive behavior, concentration problems, and pro-social behavior) was assessed via the checklist version of the Teacher Observation of Classroom Adaptation. A CR index was calculated by summing the number of risk factors to which each child was exposed. Multilevel models indicated that no main or subgroup effects were evident. These findings were largely insensitive to the modeling of CR although a small intervention effect on disruptive behavior was found when the curvilinear trend was used. Further sensitivity analyses revealed no apparent influence of the level of program differentiation. In sum, our findings indicate that the GBG does not improve behavior when implemented in this sample of English schools.