Totally aside from the fact that student and pupil can be used interchangeably in English, this seems to be an odd construction in English. Doesn't "pupil" imply that the person is a pupil at a school? If I'm naming the school, I would include it in a sentence - Jane is a pupil at City Middle School - but if I wasn't naming the school, I would just say that John is a pupil, not John is a school pupil.
The way this sentence is constructed, the most natural translation would be "Are you a student?" If the intent is to teach the vocabulary of "disgybl", you could potentially ask "Are you a pupil?" (I can imagine someone approaching mature-looking high-school student and asking if they were a pupil or a teacher), but I can't even imagine a situation in which I would ask if someone was a school pupil.
"School pupil" sounds bizarre to me. I can't imagine saying that, nor have I ever heard anyone say it. What other kind if pupil could there be? i am not British, so I am open to the possibility that it could sound natural to a speaker of British English. In American English the word "pupil" sounds somewhat archaic and insofar as is is used, it is not said of an an adult. So I agree with JimBernhar that I don't know what to make of "school pupil" in relation to a person whom I would call "chi" in Welsh. I would think "pupil" corresponds to "écolier" or "élève" in French, and an "écolier" would not be a "vous", though "élève" could have a broader application. Maybe it is just a British/American difference in usage. Learning Welsh on Duolingo is showing me that the differences between British and American usage are more subtle and numerous than I thought.
Idiomatic English should be accepted. No-one asks "Are you a school pupil" in any context. "Are you a pupil". "Are you at school" "Are you a pupil at the school" are questions that might be asked depending on context. I think the first of these correctly translates the question being asked here. "Are you a student" should also be accepted for international English.