"Uczeń" is a person attending one of "szkoła podstawowa"(preliminary school), "gimnazjum"(junior high), "liceum"(high school); "student"(pol.) is specifically for students of tertiary education(i.e those attending university or similar).
I always thought that pupils are only in Great Britain. Kids who go to school in America are called students.
True, as far as I know. But the fact that America uses the same word for kids in school and people studying at a university is very problematic from the point of view of this course, because in Polish those are totally different words.
Generally, American English is in our main answers, however if the American word is ambiguous (as here) and the British one isn't, we use the British answer as the main one. Another example: using "trousers" and not "pants".
Obviously you can translate "uczeń" to the American "student" and it should work.