Especially in high school. The are less distractions in single sex schools so the students can focus on their work.
Boys and girls learn differently and have different interests. SinglE sex schools can ensure that the learning is appreciate for their students.
There is less stigma attached to various subjects. Girls can sign up for subjects like manual arts and boys for subjects like home ec with out the fear of being the only male/female in the class.
Research have found that boys do better in literacy if it is a single sex class, and the same for girls with subjects like maths and science.
I was a math girl many years ago, so my approach will apply to the sciences. It has been said by physics professors known to me that the best way for girls to learn science is to learn it in an all-girl environment where they can thrive, and that the best way for boys to learn science is to learn it with girls, so that they can get a better perspective. We've done it the way fhat helps boys for a good long time now. It's time to let the girls learn on their own.
I believe there are benefits to attending a single sex school. For one it can at times help lessen teasing and distractions in class, when I was in an all girls class I felt way more at ease than my time in co-ed classes. In the end it would depend on the person.