I am sorely tempted to make the joke that single sex schools are bad and that it should be encouraged to have sex multiple times while at school instead.
The idea of single sex schools is to prevent the child from being distracted from their work by members of the opposite sex.
The whole point of schooling a child is to teach them, not just subjects, but about life, how to interact and how to deal with other people, of all kinds. So to remove any parts of the wider society from that place of learning can only be detrimental to the child. Children should be around as many other people of different sex, race, culture, political views, class, etc as possible to provide them with as rounded an education as possible. By forgetting that school is about learning about people as well as about subjects, then you are committing immense harm for the future of that child.
Besides which, almost anyone that has attended a single sex school will tell you that they will invariably sneak out of school and go along to the nearest school of people of the opposite sex at every opportunity anyway, so it is a complete waste of time and effort to try and keep them apart.