I started reading this story a couple of days ago, and when I wanted to continue reading it yesterday I couldn't find it among my bookmarks so I thought I'd turn to you. I think I found the story on fanfiction.net.
From what I remember, Sam and Dean where in this small town, not researching anything in particular but just passing through or something. They saw ambulances and stuff, and followed them to the local college, where a girl had just jumped from the building and died. She had this tattoo (of a pitchfork or trident or something, I can't really remember) on her lower back. While they watched the scene, I think Sam got some sort of premonition and passed out for a while. Dean then sees this ghost girl in a window, and they go to the dead girl's room to investigate if her apparent suicide was really some sort of supernatural death. They encounter the girl's roommate, who tells them that some other kids at the college also committed suicide recently. When they ask her about the tattoo, she says that the girl never had a tattoo. While Dean is alone in the dead girl's room, the ghost appears again. It grabs his arms, and he is marked with the same tattoo that the dead girl had. They then leave the room and almost get into some trouble with the police for snooping around the dead girl's room. And somewhere here I stopped reading and forgot to bookmark the story for when I wanted to continue reading it. I hope someone manages to recognize the story, because it seemed interesting and I really wanted to finish it.
ETA: Found! It's
Lessons by mmarinov. Thank you,
mellaithwen.