Fair question. My assumption is that the US schools offer students enough exposure and resources of advanced topics, to the point that top students will find their way. For instance, libraries, vast resources on the internet, community colleges, programs with local universities, and etc.

The assumption can be challenged, of course.

Case in point, this problem on Stackexchange was from a high-school homework. I think it's advanced enough for students: https://physics.stackexchange.com/questions/711174/calculati...

Getting out of doing the "boring busywork" is part of the advantage of being in the gifted class too though. Just doing both sets of work isn't a solution.

It is for grad school. Nobody in academia cares if you went there for undergrad.