[mware]

A fag could be a cigarette or from what I remember from Tom Browns Schooldays,,,one of the lower boys who does errands for 6th form students in public school.

[Westerby]

"Tom Brown's Schooldays"

Not just for 'running errands' but for relieving the hormone generated pressure from which those 6th. form-ers occasionally suffered. A tradition that was more honored in the breach than in the observance I believe, but it did give birth to the term 'silly bugger'.
I have wondered whether it was the root of the contemporary use of the term 'fag', but have never been able to find out. Any of you literary types know?