Andrew Mullins of MercatorNet explains why here.
"Lastly, there are strong moral arguments for single-sex schooling. Perhaps there was a pre-industrial, pre-urban time in the village when mixed schooling was natural and socially appropriate. But we live in a different age. A large coeducational school in the modern city can be positively damaging for some children because parents lose all management of the incipient social life of their children. Parents are the primary educators and nobody else has the right to set the moral agenda in children’s upbringing, neither a peer group nor their teachers."