The present study uses meta-analysis to synthesise findings involving 480 effect sizes from 97 studies (dated 2000-2020) to provide insights on associations between school socioeconomic status (SES) and (a) student learning outcomes; (b) schools' percentage of ethnic minority students and students' prior ability; and (c) school processes in K-12 schools. It makes three contributions to the school SES scholarship. First, it elucidates the magnitude (r = 0.58) and nature of school SES effects (e.g. larger effect sizes for achievement (vis-à-vis attainment) outcomes). Second, it clarifies the conceptual meaning of school SES, namely that school SES is less associated with school processes than it is with schools' percentage of ethnic minority students or students' prior ability. Third, the study shows that school SES is more strongly associated with specific school processes (school leadership and climate, teacher capacity, parental involvement benefiting student learning) than others (instructional programmes, educational resources).