I went to a single sex selective school in Sydney and like ls had interactions with the boys selective school next door.

I think it's the best of both worlds if your single sex school also interacts with the opposite sex. We had some great classes with just the girls and some equally great ones with the boys.

My brother went to a different all boys selective school with no sister school close by and as such they all struggled come formal time and when it was time to start dating others. We used to work together after school and he was hopeless with the girls. He did however do just as well academically and has a great group of friends still 15 years later from his school.