"Студент" in Russian is a student of a university. "Ученик" is a student in a broad sense

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So you have to use "ученица" that is ambiguous too :) "школьница" means "school student", "ученица" is more like "student" itself

Whether it's a girl or a boy

It'd be "ученица." "Ученика" is the genitive or accusative form of "ученик."