We never did Secret Santas when I was growing up, but I didn't go to Catholic schools. I don't recall when I first heard about the custom, but I'm pretty sure it was before coming to the U. of C.

I distinctly recall reading about the Christkindl custom in one of the Christmas columns of Alois Buckley Heath, one of the sisters of William F. Buckley, Jr., so that's a data point that's Catholic, German (since she used the term Christkindl) and not Midwestern.