My reunions were for various years after our small girls' academy (la de dahhh) graduated, and - at least the ones that I went to - were held at a marina restaurant at some hours such as noon to 3 pm. Those so inclined could meet at the school chapel earlier for mass. So clothes, undoubtedly still important in the I'm okay you're ok sense, were fairly simple: mostly dresses with heels or some equivalent outfit. What I'd call 'nice-restaurant' wear. I bet some portion of the attendees had had their hair done for the event, and perhaps there had been some dieting efforts before the day.
If anything, I'd say any need to worry about one's attire quickly petered out as years went by. We'd grown into who we were, so be it, though it's hard to characterize a whole bunch of individuals' attitudes.
I'd have been interested in the psychological complications of the class asking mates/husbands to join the reunion. And would enjoy being a fly on the wall at a regular sort of reunion of male and female classmates. There must have been movies or tv shows about that.
To answer your question, I don't know if there's usually a jump up in dress code - but I'm betting against it.