I don't ever remember wearing really casual clothing at the book study. Although most sisters had a hierarchy in their wardrobe. The best for Sunday and Tuesday , then the book study skirt and top and of course the service every day clothes. I guess this would have been a regional thing.
I know my husband would refuse to wear his jacket and when we attended the bookstudy in a private home, he would wear a sweater. Those were the nights that he wasn't allowed to read or pray. Well that was just fine with him