Conjecture, but it may be to focus on the native born population as a better metric for education quality since they have (presumably) been a product of that system for their entire life. Contrast that to a 1st gen immigrant who a major amount of time in a different country's system; testing them after a short stint in the US tells us much less about the US educational system. It's harder for me to think of a reason why 2nd gen should be removed, unless the assumption is the educational attainment/integration of 1st gen parents heavily biases the results of their kids. I don't know if all that holds up under scrutiny, though.

Also, because demographics aren't distributed evenly geographically, I think there is probably a case that the education system is different for different races (to the extent that racial geographic distribution is different).