Trust in Political Institutions (TIPI) is important for political stability as well as political change. We integrate two theories into the conceptualisation of political socialisations of students with and without immigration backgrounds: The theory of social capital conceptualises trust as a generalisation of positive social relationships, whereas the theory of the critical citizen implies that institutions need knowledgeable and scrutinising citizens for democratic improvements. This paper investigates the role of the school setting for political socialisation concerning TIPI using weighted German data from the International Civic and Citizenship Education Study 2016 (N = 1451 students in 59 classes; 52% were girls; mean age was 14.29; 40% of participants had immigration backgrounds). Students with immigration backgrounds reported relatively worse social relationships and bestowed less TIPI compared to their peers without immigration backgrounds. As predicted by the theory of social capital, all groups of students bestowed more TIPI if they experienced positive relationships at school to a higher degree. A weaker predictive power of civic knowledge on TIPI for students with immigration background supports notions of the critical citizen. Results indicate that social relationships are relevant for the formation of TIPI of all students, while the role of civic knowledge varies between social groups.