@Lookingformykeys
Winchester thinks by doing this it will increases Oxbridge numbers as well as school diversity. However the transition is questionable especially parents who bought in the single sex education.
Many other schools such as Charterhouse, Westminster, Rugby and Wellington have gone through this change before. In some cases such as Westminster and Wellington, they actually became better.
If you son is in VI book 2 or VI book 1, he will not even going to be studying with girls this September. None of this things will effect his Oxbridge places, it possibly going to increase his chance.
If your son is in V then it is abit more challenging but again having 19-20 girls out of 140-160 students, nothing will really change at this stage. Again your son Oxbridge chances may be going to increase because the results here is one of the best but it has less and less Oxbridge places because of being one of most elite school in the country and lack of diversity does not help ( same as Eton which has Oxbridge places dropped tremendously as well). V parents can decide to leave now but I am sure coming from Winchester with a great GCSE prediction, most of the school will take them.
MP parents just have to carry on with the GCSEs and make up their mind later after completing the curriculum.
JP this year came with the understanding of the Co-Ed situation ( they were told before they joined, I know it gave such a a short notice but they could have switched to Eton, Harrow, Radley or other all boys school during that short period) so they are most likely to stay.
I am sure in a few years the oxbridge places will keep increasing and many more leavers will go to Ivy leagues.
The school will still be one of the top independence school academically. But the atmosphere will definitely change with girl being here, for better or worse only times will tell.