학생이였어요. I had been a student? I was a student?
if you ask me "had been" and "was" are essentially the same - it's splitting hairs
체포된 사람이 학생이었어요? - Was the person arrested a student?
예. 고등학생이었어요. - Yes. He was a high school student.
For lack of better examples coming to mind:
저것은 내 가방이었어요 - That was (used to be) my handbag.
저 빨간 차는 내 차였어요. - That red car used to be mine.
저 건물은 학교였어요? - Was that building a school (at one time)?
네. 학교였어요. - Yes. It was (used to be) a school.
oh what fun.. anyway I couldn't think up any better examples -- don't sweat it -- you'll get this point by osmosis