In this multicountry analysis, we generate a student-level measure of socioeconomic status (SES) "mixing" to understand the benefits or pitfalls of placing low-SES children with diverse peers. We conduct this analysis separately for equal and unequal countries that provide the same curriculum to all children regardless of ability level, and we find some surprising similarities. For example, lower mathematics and science test scores are associated with low-SES children in mixed classrooms. We then apply this analysis to the United States, a rich but unequal country where ability-based tracking is common. For the United States, we find that the cross-national patterns are reversed for mathematics, and socioeconomic mixing is beneficial for low-SES children; however, the results for science are not significant. (Contains 4 tables and 8 notes.)