Having been listening to and reading Stalky and co. a bit and came across the following passage. The general context is that "Clewer" is being bullied horribly and Stalky and co. sort out the bullies ensuring him rest. I quote it with surrounding text to give context:
“I am pleased with my Tenth Legion. I make them my compliments. Clewer was throwing ink-balls in form this morning, instead of doing his work. He is now doing fifty lines for—unheard-of audacity.”
“You can’t blame us, sir,” said Beetle. “You told us to remove the—er—pressure. That’s the worst of a fag1.”
“I’ve known boys five years his senior throw ink-balls, Beetle. To such an one have I given two hundred lines—not so long ago. And now I come to think of it, were those lines ever shown up?”
“Were they, Turkey?’ said Beetle unblushingly.
Here "That's the worst of a fag." seems to be used in the meaning of either "the worst part of being" or in the sense that it brings out the worst. I'm assuming the former but am not quite sure. Of course the fact that Clewer was throwing ink-balls around is a good thing (i.e. he's not cowering in fear and utterly tired but rather being a spirited young boy).
What then is the correct meaning of that sentence and how should it be parsed/rewritten in non-colloquial English?
1 fag: an English public-school boy who acts as servant to an older schoolmate
(note: in the book it seems to be used to refer to all young boys not just those that are actually fags, though Clewer is fagging for the two bullies)