This study focuses on students’ subjective well-being and the association between subjective well-being and the level of academic achievement in elementary school. Despite the fact that subjective well-being in school is a significant predictor of various life outcomes in adulthood and an indicator of education effectiveness along with academic achievement, it is not sufficiently studied at the elementary school age. Complex models of subjective well-being in school usually include relations with peers, since relationships with classmates are an important indicator of a child's adaptation at school (Ladd, Kochenderfer, & Coleman, 1996) and are able to predict subjective well-being at school (Tian, Tian, & Huebner, 2016). We can distinguish two aspects of relations with classmates – cooperation and hostility, which associate with academic achievement. In this study, the level of academic achievement was examined with the SAM-math test (Student Achievement Monitoringin Mathematics) (Nezhnov, Kardanova, Vasilyeva, & Ludlow, 2015). It is a standardized assessment tool designed to measure theoretically and empirically grounded levels of mathematicalknowledge acquisition: procedural, conceptual, and functional. Subjective well-being at school was measured with the Russian version of the Brief Adolescents' Subjective Well-Being in School Scale (Tian, Wang, & Huebner, 2015), and the Russian version of the Classroom Friendship Questionnaire (Turilova-Miščenko & Raščevska, 2008) was applied to measure cooperation and hostility. The sample included 144 fourth grade students with different levels of academic achievement. One-way ANOVA indicated differences in the level of satisfaction with the school, the number of friends, the frequency of cooperation with classmates and the frequency of hostility in communication with classmates between students with a lower level of academic achievement and students with a higher level of academic achievement. At the same time, there was no difference in the affective component of subjective well-being.