Way back in the 80s when I was in highschool in my native Portugal, one of my school colleagues went to the US for a year in a student exchange program.
Now, this was a guy whose average grade in Portugal was 12 (in a scale of 1 - 20, were 10 was a pass mark).
When he came back from the US after a year he had got A grades at everything but one (were he got a B). By the way, he was no better student in the year afterwards in Portugal than before.
It always stuck with me since then the idea that highschool-level teaching standards even in quite a poor and peripheral European country were much more demanding than in the US.