it is not the subject issue ...
there are 3 parts to consider:
one is the subject: either はor が。
2nd is the topic as an object, this is marked with が
3rd is the action (if you focus on it, rather than the subject it is が).
if you ask: "Who is the student" (because you are on a theatrical play and don't know, who took which part.)
The answer might be: [大輔が学生です」
Because the topic is Daisuke, who actually plays the role. And the 「が」indicates that he is the real topic.
Same setting and you ask; "Who of you is playing the role of the student"
The answer might be: [大輔は学生です」
Because you asked for the the student. And the topic of the answer ist that the student is Daisuke.
mehhhhh
Rather difficult to explain
I hope for more answers ...