Background: National Institute for Health and Care Excellence guidelines recommend a stepped care approach for the identification and management of children with, or at risk of, attention-deficit/hyperactivity disorder (ADHD). We investigated the effectiveness, cost-effectiveness and acceptability of a group parenting intervention programme (+/- a teacher session) for children at risk of ADHD. Methods: In a three-arm cluster randomised controlled trial, 12 primary schools were randomly assigned to control, parent-only and combined (parent + teacher) intervention arms. Eligible children had high levels of parent-rated hyperactivity/inattention (n = 199). At 6 month follow-up, the primary outcome measure was the parent-completed Conners’ Rating Scale-Revised (ADHD index). Secondary outcomes included the Conners’ sub-scales (hyperactivity, cognitive problems/inattention and oppositional behaviour), the teacher-completed Conners’ Rating Scale-Revised, child health-related quality of life, parental burden and parental mental health. The cost-effectiveness analyses reflected a health and personal social services perspective. Trial Registration: ISRCTN87634685. Results: Follow-up data were obtained from 76 parents and 169 teachers. There was no effect of the parent-only (mean difference = -1.1, 95% CI -5.1,2.9; p = 0.57) or combined interventions (mean difference = -2.1, 95% CI -6.4,2.1; p = 0.31) on the ADHD index. The combined intervention was associated with reduced parent-reported hyperactivity symptoms (mean difference = -5.3; 95% CI -10.5,-0.01; p = 0.05) and the parent-only intervention with improved parental mental health (mean difference = -1.9; 95% CI -3.2,-0.5; p = 0.009). The incremental costs of the parent-only and the combined interventions were 73 and 123, respectively. Above a willingness-to-pay of 31 per one-point improvement in the ADHD index, the parent-only programme had the highest probability of cost-effectiveness. Participants found the interventions acceptable. Conclusions: For children at risk of ADHD, this school-based parenting programme was not associated with improvement in core ADHD symptoms. Secondary analyses suggested a possible reduction in parent-reported hyperactivity and parental mental health problems. Future research should compare targeted interventions against watchful waiting and specialist referral.