I'm looking for a story that had Dean as a teacher and Sam as his student (16 to 18).  The story starts in a bar or something and Dean and Sam meet and end up having sex in the bathroom.  The next day Sam gets to school and walks into class to find his new/substitue teacher, Dean, is the guy he hooked up with at the bar the night before. 
They end up once more hooking up in a bathroom at the school then it ends with Sam agreeing to run away with Dean so they can be together.

I'm pretty sure I read it on fanfiction.net before I got into J2, hence my belief that it was Sam/Dean, but its possible it is J2 and I read it later elsewhere.

I believe it was a oneshot, but I know fir sure it wasn't a long fic.