Optimal classroom climates for civic education encourage the development of knowledge, skills and dispositions necessary for students to become involved citizens. One of the simplest and most common ways of measuring classroom climate in this field is to aggregate individual students' perceptions of classroom climate to the group level; however, little has been done to assess the quality of such aggregate measures. We examined data from the United States sample of the 1999 IEA Civic Education Study to determine the reliability of aggregate measures of the openness of classroom discussion climate as reported by students. Student and class-level correlates of within-classroom variability in students' reports were considered, and aggregate reports were correlated with student outcomes. Although the aggregated classroom climate measure had only modest reliability, we identified several within-classroom sources of variability in ratings. Classrooms were also more variable in their ratings if the average climate perceptions were lower on average, and if the classrooms served students with fewer educational resources at home. However, the aggregate measures did predict a variety of student outcomes even after controlling for within-group sources of variability.