Adjusting to school contributes to the healthy introduction of all educational activities. For this reason, it is important to determine all facilitating and debilitating factors to the school adjustment process and to develop preventive studies for overcoming school adjustment. The purpose of this study is to determine the factors affecting the school adjustment of first-grade primary school students based on an ecological approach. The study group consisted of 81 teachers and 517 parents. The data were gathered for 517 children aged between 64 months and 98 months from public schools located in the central districts of Ankara in the 2015-2016 academic year. Structural equation modeling was used in the present study. Thus, according to the School Adjustment Model, externalizing behaviors, teacher-student relationship, and peer relations have a significant direct impact on first-grade students' school adjustment, whereas family involvement has no statistically significant direct influence on first-grade students' school adjustment. In addition, externalizing behaviors affect school adjustment through the mediating role of teacher-student relationship and peer relations. Also, the total effect of the externalizing behavior variable on school adjustment is -0.55. The student-teacher relationship (B = 0.53) and peer relationship (B = 0.48) variables have also had an effect on school adjustment.