Choosing a school: what are the co-ed options?
The diamond structure of schooling (mixed until 11 - single sex until 16 - mixed sixth form) is possible when single-sex schools are in close proximity. Students benefit from having time to develop at their own pace but can take advantage of a greater range of facilities. Examples include: Berkhamsted School with more to follow (for more information, look at this article from the Spectator: Insight into diamond schools)
Mixed Sixth forms allow schools to offer a breadth of subjects and to introduce a new cohort at 16+. Successful examples include: Westminster and Charterhouse (who recently announced that they will be going fully co-ed!)
Schools which remain single-sex but take advantage of geographical proximity to offer joint activities throughout the school include: Sherborne Girls partnering with Sherborne Boys and St Mary's Ascot with Eton College. Drama, music and social activities all benefit.
I am a product of a girls' school and can see the benefits of the system: not being defined by gender being an obvious example. As with all things, balance is the key and it is heartening to see our schools exploring innovative alternatives to the old systems.