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for the US student:

they went to private schools or, alternatively, they went to the wealthier public schools which teach these sort of courses, although usually on a 'tracking' system which segregates the 'smart' from the 'not so smart' at around 12/13.

for the US university (at least the more elite ones):

the above, in combination of the fact that a big chunk of the of the students are international students.