Students’ perceptions of their classroom climate have been found to relate significantly to students’ learning outcomes. The purpose of the present study was to construct an instrument for assessing elementary-school students’ perceptions of classroom climate, based on a previous instrument that was being used in Chile by a public national school mental health program as a tool for aiding teachers in improving classroom management, but which showed poor psychometric properties. We used a six-staged mixed-methods approach to construct relevant items and dimensions based on this measure and by adapting previously-existing scales. Item development included participatory construction of items involving program officials, focus groups with students, and a pilot study. The final version was administered to a sample of 6813 elementary-school students. Results showed adequate reliability and construct validity, convergent validity with school climate, and divergent validity with peer victimisation. When consequential validity was explored through semi-structured interviews with program officials and school administrators, we found that the instrument was being used as a tool for helping teachers to improve their school climate and management skills. We discuss the importance of constructing instruments using a mixed-methods approach.