:P They
could handle the thugs, perhaps (depending on how powerful/skilled they were), but that doesn't send the right message! Ren wants them alive, and also wants them to
specifically get beat up by Aoife. Once they're back in the town, word will get around that Ren von Ouless has powerful friends, and is not to be crossed, which is exactly the kind of publicity she needs! So, if Aoife protested that the guards should handle it, Ren would just wave her off and say "it has to be you, babe."
She's sure Aoife can beat them, and not only will she have the guards for backup, in case the thugs try to brute-force their way inside, but Corin as well!
She'll be fine.
EDIT: Besides, these guys are obviously criminals. I think the Academy would forgive her for using strong offensive magic in defense of a student, don't you?
EDIT 2: And yeah, I was thinking that as long as they remained outside the gates, the guards actually don't have jurisdiction. It's only if the thugs actively attempt to break down the gates and get inside that the guards would have reason to fight them off. And, the thugs aren't going to do that if they have something to focus their anger on...