Single-sex schools are better than mixed schools.
Decades ago, single-sex education was the only way to go. Then, co-education became popular as we tried as a society to strive for equality of the sexes.
One advantage to be in single-sex schools is that the effort is appreciated, there is greater competition between students and they get better marks.
On the other hand, one of the disadvantages of single-sex school is that when a student from that type of school goes to a mixed school, they can have problems with the rest of the people. For example, one student at the university who has come from a single-sex school can find it strange to fit in with the classmates and talk with the other sex because they didn't use to do it.
To sum up, I think that mixed schools are the best choice for education because students can have relations and show their point of views between them