When I was a child of about that age, the group was divided for the first time into two groups for a similar purpose. However, my teacher did not begin in the way BW describes above - and which I think is a wonderful idea. The division was simple. We practiced in the auditorium seating area. Usually, we took up the first two rows of the center section. The choir teacher simply gave one part to the first row and the other part to those in the second. But she told us that if anyone had any difficulty in reaching the notes of either part, they were to let her know and she would consider changes. There were a few changes after the first few attempts on the song. But after that no time was wasted in practicing the song.

The song, BTW, was "The Ash Grove".