That is how one of our assistant instructors does it. That is how I remember doing it back when I did Tang Soo Do in the 80s. The only problem is, there are some child black belts who do attend general classes, and while you would expect a child black belt to hold his own against an adult colored belt, in practice, it doesn't always apply when the child is less than 5 feet tall and weighs less than 90 lbs, and the adult is close to 6' tall and weighs over 200 lbs. So in that case, they have to re adjust the lines to fix obvious mismatches. Our head instructor just matches up by gender and size.