self harm can be as others have said a "control" thing. or punishment. sometimes both.

for me self harm was about feeling. sounds strange. but i needed (for almost 3 years) to cut to feel. because even though i wouldnt feel the cut until the next moring (shower) i knew that the blood meant i'd feel something at some point. as for hiding it i did hide it. for almost two years, until i rolled up a sleeve and my mum saw it. her reaction "how can you do this to me" she still has no idea why i was cutting. in my mind (very depressed at the time) i was taking it out on me so no one else 'needed to stuffer' is your DS close to this girl? if he see if she'd like to come over and talk to him - and you. offer an ear, as i know for myself that speaking to any one at school meant the risk of my family finding out- something i couldnt handle as i saw my self protecting them from what was really happening. she may open up to you about it. if you just ask her about why she wants to cut her self, if shes feeling numb or depressed. if she clams up dont keep pushing if she knows some one cares it may start that ball rolling and open her up a little KWIM?

as for the "emo - thats what they do" tag its not always right. if she's 'slipped up' and your DS has seen them - then she is less likely to be emo. - rather caught out. i only ever showed my scars/cuts to people who told me they also wanted to cut - at the time my thighs where covered in cuts/scars/scabs i'd tell them that they didnt want to stand in my shoes, it wasnt worth it.

I'm happy to talk to you about it if you like, like leasha - i too still carry scars from those dark nights.
i hope you and your DS can help her