It's not a question they can afford to take their time mulling over. Maybe it doesn't really make a difference one way or the other. In either case, they'll be seeing to some students first and others later. So, when he opens his mouth to reply, his answer is a half-blurted and largely thoughtless, "The girls'."

...There is almost certainly some Casual Well-Intentioned Sexism behind that kneejerk response, but now isn't really the time to examine that, either.