In Chile, the influence of the socioeconomic make-up of classrooms on achievement has been extensively studied in mathematics and language, but less in currently important non-traditional subjects such as civic knowledge. This paper analyses the effects of the socioeconomic composition of classrooms on students’ civic knowledge achievement in Chile, using the International Civic and Citizenship Education Study 2009 and a multilevel methodology. This research finds positive influences of higher socioeconomic intakes of 36% of one standard deviation in test scores. These results suggest that in the socioeconomically segregated Chilean education system, these effects would contribute to widening the civic knowledge attainment gap between pupils attending affluent and less affluent schools. In addition, this research finds that, on average, students with a higher individual socioeconomic status are more sensitive to these influences and that socioeconomic composition effects are more acute in the private-voucher sector than in the public sector.