I understand about not wanting "lopsided" class sizes (believe me, almost every class at my chapter was lopsided, and this year will be the hardest one!), but at the same time you don't know the specifics of when a student will graduate. Say you take on a new PNM who's a sophomore by credits, but she actually won't graduate until the year after or two years later (i.e. 5-6 years to complete their degree). Technically, they'd be in a different class based on graduation, so I think it takes a little more "digging" to determine whether or not to retain the woman to later parties.
That's just my .02
