This case study investigates, at three Swedish schools, primary school students' knowledge of the equal sign. The schools were chosen as representatives of schools whose students have different socioeconomic backgrounds. The data consist of Grade 3 and 6 students' responses to an assessment form based on Matthews et al. ("JRME," 43(3):316-350, 2012). A comparison between the schools indicates that there are great differences in the students' knowledge of the equal sign depending on which school they attend. The Swedish students perform better than South Korean students responding to the same assessment form when it comes to knowing that the equal sign does not mean an operation ("the answer to a problem"). We find this interesting to discuss, as South Korea is one of the countries that performs the best on the TIMSS and PISA.