The actual intelligence segregation is self-selected. PMC parents live together in the same neighborhoods. Whether you believe in genetic or social factors, the smart kids come from here.
In the 80s and 90s there was an educational culture that believed in finding overlooked geniuses regardless of where they are from. That narrative is completely dead. All institutions expect kids to come prepared with a resume from their youth, and have few avenues for identifying talent that doesn't meet their mold.