Retaining children will make our schools look worse than reality would dictate. The nature of any species is that there will be very successful examples, and there will be examples on the other end of the spectrum. The conservative mindset would have us believe that if we just tried harder we could educate even the lowest capability students to the point that they could gain entrance to Harvard. The child will not become brighter due to retention. The child will most likely not perform as well as the children in the class into which they are placed after retention.
Children have no choice over how capable they are.
Children have no choice over the family into which they are born. Children have no choice over the problems which life leaves on their doorsteps, and which distract from their ability to concentrate on school work. Retaining all the poorly performing children will surely impact class sizes. I see this picture of fifteen year old first grade students. It is not a pretty picture.