We started splitting up by gender in middle school for gym (NYC public school less than 10 years ago). There were no showers, but separate locker rooms, and we had to change into school shorts and a t-shirt. I think in middle school every unit was separate, but there might have been one or two sports we played with the guys.

I went to such a large high school that you had a choice of gym classes, and most were separate, but a few were co-ed. I always tried to pick things like volleyball and handball, which could go either way depending on the semester. Basketball, track, and the like were always separated by gender. Then there were classes like dance that weren't officially separated, but if one or two guys signed up for it that would be a lot (my school was pretty progressive, so it wouldn't necessarily be *weird*, just not done often).