I'm a super casual person, but I still think we should be presentable/respectable when in that kind of group setting. You'll be seeing people you may not have seen in awhile....why not put your best foot forward?
I started teaching in a time when my district required panty hose with dresses. By the time I left, the younger teachers wore chinos and sandals. Teaching is an active job.....we get on the floor and have kids throw up around us...use markers that leak ......so..... I did not wear my best stuff. Mostly khakis but I did try to cover my feet! Outside recess was very hot, too, so sleeveless ...
It was a balance to look professional while being cool and comfortable.
I believe how we look shows respect (or lack of) and we receive what we sow.