Hi again.
You asked re what year to start the girls. Obviously there is a big intake at y3 so that might be preferable as there will be lots of new people starting at once. As I said my boys started higher up the school and had absolutely no issues with settling in, however I do think boys friendships are quite different, give a group of boys a football and they will all be playing together within a few minutes. For girls there might be a benefit in being there at the start when friendship groups form. I am not worried re DD starting, but she does already have a few friends there. There is a lot more movement in and out of the school then we experienced in our local state schools so I think the new children generally integrate well in any year, but girls can be v bitchy to rach other as they get a bit older so I suppose there is a risk.
We did get advised that competition for entry to year 6 was the toughest because people try and bypass the 11+ entry. However for the girls high the junior school pupils do sit the exam so there is still a theoretical risk of not carrying on to the senior school though I think it rarely happens that a girl is in that position. The boys from the junior school don't sit the entrance exam, apart from the tp few academically who are considered for small scholarships (Think 10-20% discount). I'm not sure if there are similar scholarships for the girls high and sadly I don't think it will be relevant to DD!
I think the wakefield grammar schools are the most academic locally. There is a small but steady influx of pupils from the other local independent schools i.e. the one you have mentioned and also the one I alluded to (another mixed school, begins with S) and generally they come because their parents wanted a more academic school for them.
I am not worried re the single sex schooling. However both DH and I went to single sex schools so perhaps that affects our thinking. There is little interaction at junior school level though historically there was a mixed pre-prep (this is changing from Sept I think). At senior school level there is much more interaction - joint musical activities etc plus a lot of fraternising on the streets of Wakefield and the school buses! DSs usually meet up in mixed groups at the weekend, though it's a pain getting them into Wakefield.
There is a really lovely information morning at the start of October where you can see all the schools in the foundation. All of my children came away from those totally enthused about the schools and desperate to start. The heads of the junior schools are both fantastic and will see prospective parents at any time to talk through everything.
The entrance exam at junior level is a full day. It is tough as lots of papers but all of my three actually enjoyed the day! DD's recent report was that the level of the maths required was much easier than what we had covered with her in preparation, though we had based the prep on the list of topics from the school.
Final thought is that children do start other than in September and if you do want your child to start urgently (house move, problem at current school etc) they will arrange for them to be assessed and start pretty quickly.