My Little Pony: Friendship is Magic - Sunlight #13 by Jase1505

To be fair, when you look at the character dynamics between them Sunset is really much more of a peer and a friend she exchanges letters with, rather than a pupil towards Twilight. That and they did have the same mentor once.

So the real question is why Celestia never mentions Sunset.

Yeah, I'd be a bit curious about why you never mentioned this other student as well.