> The education system is the same for all the kids

This is incorrect, certainly an ideal we'd all like but far from reality. The educational experiences and outcomes of 1st and 2nd generation immigrants can and often does differ significantly from native-born students. Seeing the difference in the data/scores should clue you into this and helps us understand the socioeconomic impact on student achievement that immigration has. Language is often a large factor where 1st and 2nd generation students may be speaking a different language at home than they are in school. The US has significantly more immigrants than other countries in the world which is why not controlling for it skews the data disproportionately.