SUMMARY. Mental health problems are a leading cause of disability among children worldwide and in Tanzania where research has documented high rates of youth depression, anxiety, and suicide, 43% of the population is under the age of 15, and access to treatment services is limited. One theoretically important social driver of youth mental health is the primary school environment. Schools have the potential to foster cognitive, emotional, and social functioning that, in turn, promotes mental wellbeing. Further, school experiences are thought to play a critical role in shaping the adaptive systems that enable resilient functioning in youth who have experienced adversities. As such, intervening to improve school determinants holds promise as a potentially effective “upstream” prevention approach. Our understanding of what to target for intervention is limited, however, because has almost no rigorous longitudinal research has been conducted to identify specific malleable determinants of the school environment that shape mental health. Our long-term goal is to develop a multicomponent intervention that leverages positive aspects of schools to promote the mental health of youth in Tanzania. Specific aims of the study are to: (1) develop a culturally meaningful set of measurement tools to comprehensively assess the primary school environment in Tanzania; (2) examine how different dimensions of the school environment work to impact mental health trajectories of Tanzanian youth; and (3) determine how features of the school environment can be leveraged to buffer the impacts of exposure to childhood adversities (violence exposure and deprivation). To accomplish these aims, we will collect data from teachers, caregivers, and students in 60 randomly selected primary schools in the Mwanza region of Tanzania. For aim 1, we will develop a robust set of observational and survey measures of the school environment, using qualitative research to investigate cultural relevance and a psychometric study to examine the validity and reliability of the adapted measures. To accomplish Aims 2 and 3, a random sample of 50 5th grade students in each of the 60 selected schools (total n=3,000) will be surveyed at 6-month intervals over 3 years for a total of 6 assessment waves. Observational assessments of schools and caregiver and teacher surveys will enable multi-informant assessment of key constructs. Longitudinal mediation models will assess the influence of school environment factors on mental health and examine emotional, cognitive, and social processes as explanatory mechanisms (Aim 2). Moderated mediation models will determine whether the negative mental health impacts of adversities are buffered for children in positive school environments. Qualitative interviews with parent/child dyads will provide contextualized information about these processes (Aim 3). We expect the study will have a positive impact by identifying factors in the school environment that can be target...