I think single-sex is dangerous for 2 situations. First in Muslim countries they applied this system for a long time and still some countries make it a must, it only raised rapists and men that see themselves superior gender. We have to accept we still live in a patriarchal world so these are the facts.
Second, schools are not only to teach lessons about math, chemistry or physics, children have to know how to live and get along with opposite sex. They need to observe what they are going through.