Kids raised in Potemkin-village environments, escape and make contact

Can anyone ID this story? Ca. 1975 I read a YA science fiction novel, which was something like the following. In the future, racial/genetic differences have been gradually lost due to intermarriage, and almost everyone is of the same generic ethnic type, e.g., everyone's skin is sort of the same cafe-au-lait color. However, a small number of humans have been maintained as living artifacts of the historical ethnic groupings: Caucasian, Asian, African, etc. The protagonists are kids of these types, who have been raised in fake Potemkin-village environments that simulate the 20th century. All the adults and other kids are actors who wear makeup. The kids start to notice that something is wrong, and secretly manage to get in contact with one another through hidden messages in letters. They escape from their fake environments and travel around the world in search of the truth and their own destiny.