Research to date on the effects of between-school tracking on inequalities in achievement and on performance has been inconclusive. A possible explanation is that different studies used different data, focused on different domains, and employed different measures of inequality. To address this issue, we used all accumulated data collected in the three largest international assessments—PISA (Programme for International Student Assessment), PIRLS (Progress in International Reading Literacy Study), and TIMSS (Trends in International Mathematics and Science Study)—in the past 20 years in 75 countries and regions. Following the seminal paper by Hanushek and Wößmann (2006), we combined data from a total of 21 cycles of primary and secondary school assessments to estimate difference-in-differences models for different outcome measures. We synthesized the effects using a meta-analytical approach and found strong evidence that tracking increased social achievement gaps, that it had smaller but still significant effects on dispersion inequalities, and that it had rather weak effects on educational inadequacies. In contrast, we did not find evidence that tracking increased performance levels. Besides these substantive findings, our study illustrated that the effect estimates varied considerably across the datasets used because the low number of countries as the units of analysis was a natural limitation. This finding casts doubt on the reproducibility of findings based on single international datasets and suggests that researchers should use different data sources to replicate analyses.