Through most part of the 20th century lots of school only offered education for one gender, but from the 80s this has changed. Is it an evidence of progress?
The advantages of single-sex schools are that the students are likely to be more focused on their studies, they usually get better marks and the teachers (who have to be of the same gender) can teach them easily a more personal education.
However, all these schools are private, and there are some problems that they cannot solve, but mixed, public schools can. For example, some students at university who have come from a single-sex school find it strange and even uncomfortable to share the classroom with the other sex. That is not a problem with mixed schools, where equality is an essential value. Otherwise, boys and girls don’t pay attention to each other until secondary school.
To sum up, I think that mixed schools are the best choice for education. Nevertheless, it is not a big problem that they go to a single-sex school until they are 12 or something like that, and then they should go to a mixed high school.