The longitudinal research project “Development of academic and social competence of primary school pupils in classrooms differing by their multilingual composition” (KEIMS-plus) supported by the German Research Foundation (DFG) investigates the development of social, academic and linguistic competencies and their interrelations during the early school years of nearly thousand students. Schooling can be seen as a major developmental and learning opportunity for social competence because both interacting in classrooms and receiving feedback on social behavior foster not only students’ development of social skills but also the acquisition of social knowledge and the development of self concept of their respective abilities. Concerning the assessment of social competencies several approaches used by different research disciplines could be identified. The study KEIMS-plus refers to three perspectives: aspects of social information processing (social-cognitive interview following Mayeux & Cillessen 2003), components of social self concept (SCSocComp, Arnold & Lindner-Müller, 2007) and prosocial behavior (SDQ, Goodman 1997). In this respect the measurement models of the applied instruments for assessing social self concept and prosocial behavior will be presented for the first three measurement points.