Students' individual learning is supposed to be based on cognitive and social processes. Therefore, students' social skills are assumed to play an important role for school performance. This study set out to investigate the links between students' peer relations skills and assertion skills and their grades for written performances and oral performances. In the middle of the school year, 94 ninth-grade students of a German secondary school reported on their peer relations skills and assertion skills. Additionally, at this point of measurement, students' initial school performance in German and mathematics was measured by standardized academic achievement tests. In these two subjects, students' grades in in-class tests and for oral participation were collected during the following 5 months. These grades were given by the students' respective subject teachers. Path analysis revealed that students' peer relations skills were positively associated with their grades in in-class tests, but they were not linked to their grades for oral participation. In contrast, students' assertion skills were positively related to their grades for oral participation, whereas no correlation was found between students' assertion skills and their grades in in-class tests. These results are discussed with respect to the role of peer relations skills and assertion skills for students' academic learning and the fostering of these skills in the classroom.