BackgroundPoor mental health in childhood is common, persistent and associated with a range of adverse outcomes that include persistent psychopathology, as well as risk-taking behaviour, criminality and educational failure, all of which may also compromise health. There is a growing policy focus on children’s mental health and the role of schools in particular in addressing this.ObjectivesTo evaluate whether or not the Incredible Years^®(IY) Teacher Classroom Management (TCM) training improved children’s mental health, behaviour, educational attainment and enjoyment of school, improved teachers’ mental health and relationship with work, and was cost-effective in relation to potential improvements.DesignA two-arm, pragmatic, parallel-group, superiority, cluster randomised controlled trial.SettingA total of 80 UK schools (clusters) were recruited in three distinct cohorts between 2012 and 2014 and randomised to TCM (intervention) or teaching as usual [(TAU) control] with follow-ups at 9, 18 and 30 months. Schools and teachers were not masked to allocation.ParticipantsEighty schools (n = 2075 children) were randomised: 40 (n = 1037 children) to TCM and 40 (n = 1038 children) to TAU.InterventionsTCM was delivered to teachers in six whole-day sessions, spread over 6 months. The explicit goals of TCM are to enhance classroom management skills and improve teacher–student relationships.Main outcome measuresThe primary planned outcome was the teacher-reported Strengths and Difficulties Questionnaire Total Difficulties (SDQ-TD) score. Random-effects linear regression and marginal logistic regression models using generalized estimating equations were used to analyse outcomes.ResultsThe intervention reduced the SDQ-TD score at 9 months [adjusted mean difference (AMD) –1.0, 95% confidence interval (CI) –1.9 to –0.1;p = 0.03] but there was little evidence of effects at 18 months (AMD –0.1, 95% CI –1.5 to 1.2;p = 0.85) and 30 months (AMD –0.7, 95% CI –1.9 to 0.4;p = 0.23). Planned subgroup analyses suggested that TCM is more effective than TAU for children with poor mental health. Cost-effectiveness analysis using the SDQ-TD suggested that the probability of TCM being cost-effective compared with TAU was associated with some uncertainty (range of 40% to 80% depending on the willingness to pay for a unit improvement in SDQ-TD score). In terms of quality-adjusted life-years (QALYs), there was evidence to suggest that TCM was cost-effective compared with TAU at the National Institute for Health and Care Excellence thresholds of £20,000–30,000 per QALY at 9- and 18-month follow-up, but not at 30-month follow-up. There was evidence of reduced disruptive behaviour (p = 0.04) and reductions in inattention and overactivity (p = 0.02) at the 30-month follow-up. Despite no main effect on educational attainment, subgroup analysis indicated that the intervention’s effect differed between those who did and those who did not have poor mental health for both literacy (interactionp = 0.04) and numeracy (interactionp = 0.03). Independent blind observations and qualitative feedback from teachers suggested that teachers’ behaviour in the classroom changed as a result of attending TCM training.LimitationsTeachers were not masked to allocation and attrition was marked for parent-reported data.ConclusionsOur findings provide tentative evidence that TCM may be an effective universal child mental health intervention in the short term, particularly for primary school children who are identified as struggling, and it may be a cost-effective intervention in the short term.Future workFurther research should explore TCM as a whole-school approach by training all school staff and should evaluate the impact of TCM on academic progress in a more thorough and systematic manner.Trial registrationCurrent Controlled Trials ISRCTN84130388.FundingThis project was funded by the National Institute for Health Research (NIHR) Public Health Research programme and will be published in full inPublic Health Research; Vol. 7, No. 6. See the NIHR Journals Library website for further project information. Funding was also provided by the NIHR Collaboration for Leadership in Applied Health Research and Care South West Peninsula (NIHR CLAHRC South West Peninsula).