[GoLightly]

Other than adult catechism classes for converts & unbaptized adults, I don’t think I’ve ever heard of adult Sunday school. I think most Protestant churches have Bible study groups & maybe some of them meet on Sundays, but they aren’t “Sunday school”. Sunday school is for children.

[maryz]

It may be outdated, but I've read of it -- I think in some of L.M. Montgomery's books (Mrs. Lynde I'm pretty sure typically went to Sunday School), and in Life with Father (and probably others that escape me at the moment) -- all older books, you'll note. The practice IIRC was to hold Sunday School after church.