To assess the associations between social and physical school environments and the prevalence of traumatic dental injuries (TDI) in 12-year-old children in Thailand. A cross-sectional study in 52 urban schools in Thailand was carried out from a sample of 2,725 12-year-old children that were clinically examined for TDI and interviewed. Cluster analyses were performed to classify the schools into supportive and non-supportive schools by social and physical environmental characteristics. Analyses of the associations were performed using multilevel analyses, accounting for school variations and controlling for confounding factors at the child level. 35.0% of children had TDI. Prevalence was twice as high amongst boys than girls. The prevalence of TDI was significantly lower in the schools with a supportive social environment (Crude OR = 0.6 (95% CI = 0.4 to 0.8, p = 0.004)). The adjusted OR was 0.7 (95% CI = 0.5 to 0.9, p = 0.02). This statistically significant association existed in boys but there was only an insignificant tendency of association in girls. There was no statistically significant association between TDI and the physical environment of the schools. But there was an insignificant tendency of association with the physical environment in girls. TDI were much more common in boys than girls. TDI were significantly less prevalent in male children in schools with supportive, compared to less supportive social environments. In boys, there was a tendency for the more socially supportive environment to be more protective rather than the effect of any type of physical environment. In girls, this protective tendency was only apparent when school environments were both more socially supportive, and physically favourable. Keywords: dental, injuries, determinants, schools, sex