My son went to an all-boys selective secondary that expanded and amalgamated with the selective girls' school about halfway through his school career.
When it was an all-boys school the boys did everything - arts, cooking, humanities, music, and they had superb choirs, orchestras and musical groups. As soon as the girls joined, gender stereotyping began, and almost all the boys dropped out of music and the choirs, especially, the standard of which dropped dramatically.
My son, a keen musician and singer, in later years toured all the classes with the very popular barbershop group to try to recruit more boys to the choirs, and out of a school of 1100, only 2 boys joined!
I used to be a fan of co-ed, but my opinion has changed and I now think boys, especially, need a protected environment to flourish and be themselves, as education nowadays is so slanted towards girls in pace and learning style.