Kurt went to Dalton to escape Karofsky's bullying, as in the show; but while he's there, Karofsky's therapist approaches him about the possibility of the two boys writing to each other. Kurt resents it at first, but soon comes to enjoy communicating with Dave.
Update: found by the superb scififreaker. (The story is Dumb by nonexistantpup.)
Key features/plot points:
- Dave is far dumber than he's portrayed as being in the show. His spelling is atrocious, for a start.
- Blaine sympathises with Karofsky because he feels guilty about how he treated a transgendered student at his previous school.
- The "letters" from Karofsky include his homework assignments from his therapist, at least at first, which means Kurt gets some intimate and very embarrassing insights into what Dave thinks about him.
- Dave's assignments are full of crossings out, but nothing is erased, which is a deliberate part of his therapy.
- One of the first questions Dave wants to ask is, concerning Blaine, "is he your boyfriend?"
- It is not the one I keep finding where Dave writes half-assed letters to everyone at the beginning of the story. Kurt is directly approached by Dave's therapist, which is unlikely, unprofessional and crucially changes the plot.
- Kurt and Dave never meet; they do have a conversation on the phone towards the end, however, and Dave accidentally terrifies Kurt into hanging up.
- It was, if I recall, split into two parts, maybe three. It was not a long, multi-chapter epic, though each part was quite substantial.
I really hope someone recognises this story, I'll be so grateful!