This study reports a meta-analysis on the effects of ethnic minority share in school on achievement test scores. Best evidence from the studies that have appeared thus far on this topic shows that these compositional effects appear small in general, but may be larger when the ethnic minority group is African Americans in the USA than when the minority group consists of immigrants. A high share of students from an ethnic minority group seems to affect the achievement of students belonging to the same ethnic group more than the achievement of students belonging to the ethnic majority or to other ethnic minority groups. Effects of the share of immigrants on test scores of ethnic majority students even seem to be close to zero. Several robustness checks confirm our results. The review concludes with a discussion of implications for research and policy practice.