I see part of the problem is that you heard S has a crush on you from someone else, so it's hard to be direct to S (I know engineer mentions being direct with D and I can see doing that).
You could be the one to tell S that you have a crush on D, but I don't feel right about that either. D should be the first one to know that you do.
Does S show you by his behavior that he has a crush? Then you could just tell him you only want to be friends. Not easy, but clear.
As I got older, even in my twenties, I tried to be clear; I was and am not always great at this. Plenty clear when interested, though.