Well, let us look at this again. And not as per what nationality is better.
What I belive is this. Some Immigrants, no matter where they are, tend to do better in school than natives. This is, of course, a generalisation, so there will be exceptions.
I believe some do better because they come from countries that may not be as develped, free or what ever the reason for their immigration may be, hence they know that they must work hard to get the lifestyle they desire. Ergo, they make their children work hard.
In oppostion to this, some natives have had it good all along, so the need for betterment is not that pronounced anymore.

A generalisation, but I think there is a lot of truth to it.

What comes around, goes around....