While numerous studies have reported the effect of school socioeconomic status (SES) on achievement, the factors that can cause this relationship are not well established. This study is, therefore, an attempt to understand school SES and students' mathematics achievement relationship by assuming that this relationship occurs through a correlation between school SES and school characteristics. Identifying these school characteristics is crucial to reduce the relation between SES and achievement for educational equity. Focusing on the 8th-grade mathematics data from Trends in International Mathematics and Science Study (TIMSS) 2015, this study aimed to identify school characteristics (quality of mathematics teaching at school, discipline at school, sense of school belonging, and school academic emphasis) that can mediate the relationship between school SES and students' mathematics achievement. The results of multilevel regression analyses showed that controlling school characteristics reduced the relationship between school SES and students' mathematics achievement in most of the educational systems. However, the results of multilevel multiple mediation analysis showed that the relationship between school SES and students' mathematics achievement were mediated through discipline at school, school academic emphasis, or sense of school belonging in some educational systems. In addition, the results indicated that the quality of mathematics teaching at school was not a mediator in this relationship. These results suggest the need for eliminating the effect of school SES on some school characteristics to improve equity in education.