Who are those girls? They are students.

A: Who are those girls?
B: They are students.
(my bold, A Guide to Teaching Junior High School English, by Ting-chi Tang, p375)
Is the above dialog acceptable to native speakers?

Yes.

A: What are those girls?
B: They are students.
(my bold, by a Taiwanese teacher of English)
Is the above dialog acceptable to native speakers?

It's possible.