Grouping students into different classes based upon their academic achievement or ability is a relatively common practice across the world, particularly in English-speaking countries. Yet some argue that teaching students in mixed-ability classes is a better approach, with important benefits for those from disadvantaged socio-economic backgrounds. Unfortunately, existing evidence comparing outcomes across these approaches is scant. This paper hence provides new international evidence on this matter, drawing upon data from the 2015 Trends in Mathematics and Science Study study. Our results point towards null effects; neither approach leads to demonstrably better outcomes for students than the other. This also holds true for young people from disadvantaged socio-economic backgrounds, suggesting that between-class ability grouping does not harm the educational prospects of these children. Some limited evidence does emerge, however, that primary school teachers believe they are better able to support struggling students—and challenge the highest achievers—when ability grouping is used.