There are two sides as to why anyone would favor either.
Equality:
As a default, if two things are different in any way, the other one is some way better. Since both genders have to be treated equally, it's easiest just teaching them the exact same things at the same time. That way not only do they share the curriculum, but also the environment and so on.
The counter-argument is that boys supposedly can't focus with girls around. Or, alternatively, the girls won't be able to focus because the boys won't let them. Both are highly sexist, but sadly, statistically they probably work to a degree.
the counter-argument to the counter-argument (I can only keep a stiff upper lip about this because I'm tired as hell) is that a balanced social life actually helps people, or at least the kinds of people high schools want: compassionate students.
financial efficiency:
pretty simple really. Even if you have to have two mixed classes instead of a girls and a boys one, you'd need measures to counteract possible differences in the numbers of each gender, probably different buildings or classes at the very least for the genders, and maybe even different teachers, since a male teacher may be prone to look down on the female students, at least if the society is jilted enough to bastardize equality in such a manner.
On a side note, why do I always start writing my posts with, for one, "there are x sides" and secondly, x is always n-1, n being the original number of sides I thought there