-가

Compare 학생이가? with 어느 학교 학생이고?. The first checks whether someone is a student; the second requests information about which school.

This sentence-final -가 is not the subject particle 이/가. Read the whole predicate: a subject marker inside a sentence does not turn the sentence into this question.

The bare present noun predicate uses -가. With past-tense material between the copula and the ending, use the -나 family instead: 학생이었나?. This is a familiar-speech ending, not a shortcut for polite Korean. Respectful questions can keep the shared standard constructions.