I don't remember much, but it's an older fic, on the longer side. Definitely high school. Sam (possibly girl Sam) tells some kids at school that he's dating Dean, but he isn't, it's before anything wincesty happens between them. He asks Dean if he'd play along and Dean agrees. It's a slow build, filled with fluffy schmoopy scenes, stuff like Dean walking Sam to class and carrying his books, holding hands etc. They start to get real comfortable 'pretending' and finally admit they don't want to pretend anymore and become a real couple. Pretty sure John's away the whole time. I might be getting another fic mixed up with this one, but I think there is a bonfire at some point, for home coming maybe.

I'm sorry I don't have more to go on. :(

FOUND Becoming What We Pretend To Be by locknkey