BackgroundThere is recognition that health and education are intrinsically linked through, for example, WHO's Health Promoting Schools (HPS) framework. Nevertheless, promoting health via schools is seen by some as a zero-sum game—ie, schools have nothing to gain, and may experience detriments to the core business of academic attainment because of focusing resources on health. Crucially, there is a paucity of evidence around the impacts of health and wellbeing policy and practice on attainment, with recent Cochrane reviews highlighting this gap. This study explored the zero-sum game hypothesis among schools with varying levels of deprivation—ie, the role of health and wellbeing interventions in schools in reducing or widening socioeconomic inequality in educational attainment. MethodsWales-wide, school-level survey data on health policies and practices, reflective of the HPS framework, were captured in 2016 using the School Environment Questionnaire. Questionnaire data were linked with routinely collected data on academic attainment. Primary outcomes included attendance and attainment at Key stages 3 (children aged 12–14 years) and 4 (15–16). Interaction terms were fitted to test whether there was an interaction between free school meals, overall HPS activity, and outcomes. Linear regression models were constructed separately for schools with high uptake of free school meals (>15% of pupils) and low uptake (<15%), adjusting for confounders. FindingsThe final analyses included 48 schools with low uptake of free school meals and 49 with high uptake. Significant interactions were observed between free school meals and overall HPS activity for Key stage 3 attainment (β=0·28, 95% CI 0·09–0·47) and attendance (0·05, 0·02–0·09), reflecting an association between health improvement activities and education outcomes among high, but not low, free school meal schools. There was no significant interaction for Key stage 4 attainment (0·18, −0·22 to 0·57). InterpretationOur findings did not support the zero-sum game hypothesis; in fact, among more deprived schools, there was a tendency for better attendance and attainment at Key stage 3. Schools must equip students with the skills required for good physical and mental health and wellbeing in addition to academic and cognitive skills. The study included a large, nationally representative sample of secondary schools; however, the cross-sectional nature has implications for causality. FundingSupported by the Centre for the Development and Evaluation of Complex Interventions for Public Health Improvement (DECIPHer). Jointly funded (MR/KO232331/1) by the British Heart Foundation, Cancer Research UK, Economic and Social Research Council, Medical Research Council (MRC), the Welsh Government, and the Wellcome Trust, under the auspices of the UK Clinical Research Collaboration. GFM's time for analysis and abstract preparation was funded by an MRC population health scientist fellowship (MR/K021400/1).