Social relations between students and their peers have not been studied more extensively in our expert scientific literature, and performed studies have rarely focused on the association between these relations and school achievement. Comparison of the quality of social relations and school achievement at different ages has been especially neglected, which is the basic goal of this paper. The sample comprised 235 fifth-grade students and 230 eighth-grade students from nine (18 in total) classes in each grade, selected from four schools from the narrow Belgrade area. Besides students, research also comprised their class teachers, whose number corresponded to the number of classes that students came from (18). Twenty-seven variables were entered in correlation matrix for fourth- and eighth-grade students, and factor analysis yielded nine factors, which were only slightly different between the two ages. In both cases, acceptance by peers and orientation towards peers had the biggest importance in explaining the total variability. The factors in multiple logistic regression which determined the school achievement of fifth-graders to the largest extent were their acceptance, parents' characteristics (educational level and employment) and social adaptation. When it comes to eighth-grade students, educational level of parents and social adaptation are directly correlated with school success, and aggressiveness is inversely correlated with this variable.