%-)Hmm ... I wouldn't worry about cancer at your age. The odds are statistically zero. ;-) It could be your laundry deterdgent if you've been using the same kind since then ... maybe your bra is too tight or something. Is it both nipples? You could have an infection maybe ... then the doctor could put you on some medication. If it's been there since 6th grade and you're now in ... 8th? 9th? Well, it's been a few years, and you've had no other bad symptoms that would really motivate you to go to the doctor, I would assume it's fine.
I'm probably not much help, sitting here mumbling, but I would say don't worry about cancer now; start worrying about it about 30 years from now:-)
Hope I helped a little!
i also have had the same problem, but its happened to me way before i even started wearing bras. i just noticed that the skin was white-ish so i just peeled it off. i've done that for the rest of my life and it became normal to me. when i became aware that most adult nipples, dont usually do that, i figured that it might stop after my breasts were fully grown. right now im gonna be 18 in a month aand...i dont think they're gonna get and bigger...they still are able to peel off and im starting to think there is something wrong o__o