What is the role of Japan こちらがのさんですが here? Can it be replaced by は?

Can be used, and the literal meaning of the two is the same, but considering the context, it is likely that "が" is more appropriate.

1, "が" (implicitly) emphasizes the theme (emphasizing that Nakata has the status of a transfer student, not others)-You see, when asking questions about the theme, you can only use "が".

"はは" is a simple statement without other meaning. What it emphasizes is that what is to be said next is the key point of this sentence. Whether this person is here or there; Whether he is Nakata or Tanaka, in short, we are talking about transfer students, not students who fail-this is the theme)

2, because 1, using "が" is more respectful to others (this seems to be specially introduced to everyone for Nakata's sake, not to say common sense known all over the world).

There are many subtle differences in Japanese. To put it bluntly, it means talking to people and talking nonsense with ghosts.