The reason schools put children of the same age into a single class is because today’s educational system was developed during the Industrial Revolution to provide outputs suitable for the world of standardized mass-production. We usually forget that for all of human history and across all human societies, education was conducted in mixed-age groups in which older children helped younger children. This led to older children developing suitable emotional and moral qualities and to younger children seeing these qualities evinced so that they, in turn, would be ready to adopt them as they grew older.
Conversely, classes full of same-age children are horrific: they amplify the weaknesses of the child while providing no examples of more adequate ways of being in the world. So the idea that a child’s social development will suffer if it is taken out of a class comprising children it’s own age is in fact the precise opposite of reality. Moreover, highly intelligent children are more aware of the fact their peers are mostly savage little animals; moving them up a few grades usually results not only in greater intellectual stimulation but also a profound sense of relief.
We really need to get past our fixation with an industrial-age approach to one-size-fits-all education and focus more on what the child’s mind and personality really benefit from. Until then, gifted children will continue to be placed incorrectly because of incorrect assumptions, and slow children will forever feel inadequate.