A group of children walking in pairs, that is, or is the term old-fashioned now? I've only heard it in books about schools, especially the Chalet School "...or I'll have to make you croc!" I don't remember teachers ever saying it for real, although we frequently did walk in twos on school outings, and there would be the arguments about holding hands.
We never wore hi-vis though, as children seem to have to do these days.