The comparative analysis of the causes of class-specific unequal educational opportunities in the general education system in Germany and Canada reveals that Germany pursues a highly selective institutional design of its education system, which particularly disadvantages children from educationally disadvantaged backgrounds. The comparison with the handling of heterogeneity in the education system in Canada—where class-specific disparities in opportunities are particularly low and overall educational performance is high—provides fruitful insights for improving class-specific educational disadvantages in Germany. It becomes evident that the significantly better educational opportunities for children from lower classes in Canada are primarily due to a complex and interconnected bundle of institutional causes and characteristics of the teaching staff.