vacua, I'm glad she seems ok physically.
The reason I asked you those questions is because, ime someone binging and purging is feeling 'out of control' and aspires to be 'in control' - so it is much more like bulimia than anorexia - in which case, if there is a trigger that sets her off, perhaps you can change that situation - so for example, if it was me, it would be being alone in the house in the afternoon, so perhaps change that scenario so she isn't, or perhaps it's when she gets home from school - maybe you and she could make a regular thing that you do. Or perhaps it's certain foods - so don't have them in the house, have 'healthy' foods in the fridge - easily eatable things like carrots ready cut into sticks, grapes, cheese cubes etc, so she may not feel the fullness and need to purge.
THe status thing among friends is a total bugger. It is often like that and is a real nightmare - bizarrely I think it is often because they too would 'like' the kudos of an ed.
From what you've said she really does sound like a 'low weight bulimic' rather than an anorexic - when I was a student I was borderline anorexic, and although I didn't ever make myself sick, I would purge in other ways, but always with a minute food intake. The thing with anorexia is control - controlling your food intake, I think bulimia is like fulfilling a need for something, and then guilt overtakes you. My bf had bulimia (we were a good pair!).