Peer relationships in lower-secondary classrooms play a crucial part in students' academic and personal lives. This study uses social network analysis to investigate aspects influencing formation of both likeability and antipathy ties between students in Czech lower-secondary schools, with a special focus on the role on socioeconomic status. Data and research design employing exponential random graph models (ERGMs) allow researchers to explore roles of SES, gender, and several other structural network variables simultaneously. Using cross-sectional data from 435 students in 21 classrooms, this study suggests that high-SES students tend to receive more likeability ties and less antipathy ties compared to others. The overall results do not suggest a tendency of students to give preference to same-SES peers, however, SES homophily was found significant in 2 of the 21 sample classrooms. Additionally, this study confirms the effects of gender homophily, mutuality, transitivity, and preferential attachment on formation of peer relationships. The effects of SES seem to be related to the effect of mutuality, with networks with high mutuality effect not influenced by the effects of SES.