I think it's a shame that there isn't a girls' sixth form in Richmond as a state option, given that there is clearly an appetite for a girls' school below this level. I don't understand what the benefit to the girls is of admitting boys. If there isn't a benefit, what is the point? There are lots and lots of coed sixth forms already and girls at Waldegrave who want to be in a coed environment have plenty of other choices.
I also don't quite understand why Tiffin wants girls in their sixth form. Any parent or child who prefers a coed environment has much more choice than anyone who wants a single sex environment in the state sector and can achieve this easily by choosing an actual coed school.
And why do the boys need reassurance? If you choose to attend a school largely composed of the other sex, you need to be confident in your choice, clear about why you want to attend and able to deal with the consequences. If you need to be reassured, it is probably the wrong school for you.
None of this directly affects me but I would not be happy with it if I'd sent my daughter to Waldegrave in the expectation of a single sex education, tbh.