There were studies made in India that showed otherwise. Without segregation the teachers were teaching to the level of the smartest students in the class, so the slower students were being left behind and became unmotivated. With segregation, the teachers were still teaching to the level of the smarter students in the class, but that level was much closer to the level of the slower students so they could catch up.

Keep in mind the actual reality of the situation might be different in each country, so it's not like one set of studies was necessarily wrong. Maybe teachers in the US teach to the level of the average student, so the results would be different there.