This study examined the friendship processes around prosocial and aggressive behaviors and the moderating role of early adolescents' perceived relatedness with teachers. Differences between fifth graders in elementary school and sixth graders in middle school were examined. The sample was from 48 classrooms (N = 879, 51% girls at Wave 1, N = 859, 51% girls at Wave 2; 27 fifth-grade classrooms and 21 sixth-grade classrooms). With longitudinal social network analysis (RSiena [Simulation Investigation for Empirical Network Analysis]; see Ripley, Snijders, Boda, Voros, & Preciado, 2017), we found that friends were similar to each other in prosocial behavior, as well as aggressive behavior, and this similarity was due to selection, as well as influence effects. In general, there was a preference for prosocial friends as they tended to receive the most friend nominations. Perceived relatedness with their teacher amplified students' preference toward prosocial peers as friends, as well as decreased students' preference for aggressive peers as friends. Friendship processes and the moderating role of relatedness with one's teacher showed some differences between fifth and sixth graders. Sixth graders in middle school were less attracted to prosocial peers as friends and showed higher overall prevalence of aggressive behavior compared to fifth graders in elementary school. The moderating role of relatedness with their teacher on friendship preference toward highly prosocial and less aggressive peers was stronger for sixth graders compared to fifth graders.