This study reveals, from a middle school student perspective, the social support from parents, teachers, and friends, as well as the structural relationship of school adaptation and grit. Moreover, it explores the direct and indirect effects of social support on school adaptation, focusing on the mediating effect of grit. For this purpose, the structural equation model was applied to Wave 3 of the Korean Children and Youth Panel Survey 2018 (KCYPS 2018) data set (9th grade, N = 2,320), conducted by the National Youth Policy Institute. The results are as follows: First, parental support perceived by middle school students did not significantly affect school adaptation. Second, teacher support perceived by middle school students had a direct and significant positive effect on school adaptation. Third, the effect of teacher support perceived by middle school students on school adaptation was partially mediated by grit. Fourth, the support of friends did not have a significant effect on school adaptation, but in terms of impact on school adaptation, it was found that grit had a fully mediating effect. Based on the findings, support and countermeasures are presented to improve the level of school adaptation of middle school students.