Net immigration has been trending down for a decade, but I'm not cynical enough to think we're not churning out some pretty smart kids of our own!

Perhaps, but the percentage of Americans foreign-born is at a 100 year high. And the percentage of under 18s who have a foreign-born parent is at an all-time high (25.6% of students, the previous peak was 21.6% in 1920).

And if their children are underperforming in schools it would be important to know.