That post is a little lengthy, so I'll summarize it here. I wore a long, casual skirt and a sweater (it was February) and I didn't feel out of place. BG said it though - it ranged from jeans to sparkly cocktail dresses.
Of course, it really depends on where your SO's class is having theirs. My boyfriend's class had theirs at the Officers' Club, but I've heard of other classes having theirs at Maguires (which is a bar). I think that would change the attire...
