Mine are being privately educated. 6th form is a few years away but we will most likely return to the state system when the time comes. This is of course assuming that they can get into a good state school.
By the time a teenager is 16 the school's job is done. If by this stage he/she hasn't developed the necessary study skills and self discipline to do well at A level regardless of school attended then he/she is going to struggle once at university.
In the case of DS his school sends a lot of 6th formers to Oxbridge. Many of his teachers are alumni and they tutor pupils for the aptitude test and for the interviews. But then the £15k pa saved will pay for enough tutor hours and leave me with some change
Another thing to consider is that the popular universities are being pressured to accept more state school students so if they have two equally impressive applicants and one is from a state school and the other is from a private school......
Sounds like she's doing really well and there's a good chance she'd do well either way, academically. Unless there is a good reason why she might not be as happy at the grammar, it sounds like your sacrifices would not be worth while.
Just as others here say - the specifics matter most and much more than my general answer above.
Consider travel convenience (can she do homework on the bus/train etc, how long is it), the time to leave the house and therefore the time she will have to get up (factor in hair etc!), length of the school day, the individual subjects she wants to do with the examination boards of the subjects, the sport, drama, DoE/World Challenge opportunities, the university preparation, the social life, how many newbies come in at year 12 etc?
Is one or both single sex? What were the teachers/facilities/HT/other pupils like at open evening?
Then compare and see what is important to you and your DD.
It will be down to the specific schools. Presumably we're talking about 6th form here.
Do both schools cover the subjects that she wants to do? What are the class sizes for each? And the facilities for those subjects?
Do she have thoughts about uni? If so what support is given by each school (and ime this is where private schools can give an edge as they are entirely focussed on this, rather than on league table results)? What is their program for uni admissions like, what info and support is given to the pupils if they face interviews or additional tests eg BMAT.
I would also be looking at movement at 6th form as well. Good schools do everything possible to keep their pupils and have less movement at 6th form. A school which lost a lot of pupils to somewhere else would raise doubts.
My daughter has been in state school until 15 years - and we're now lucky that she has been offered a 50% scholarship to a good local private school. However we have also heard from admissions direct, that she will most likely get a place at a famous top grammar school. She's already confident, so that's not really the issue and both schools are academically great (the grammar actually rates higher). We're borderline for any bursary to boost the scholarship, so we would have to make pension sacrifices to pay the rest of the private school fee - (15k for two years) is it worth it? In other words, do you think in this day and age, private school education still counts in the work place/social scene? I am torn between giving her a 'polish' after years of state, balanced against the worry that she will be unable to afford oversea invites from friends, and school trips, which will affect her assurance in reverse!! Am also wondering if at top Uni's, there is still a snobby attitude and wonder if she should experience the best of both school worlds so she can handle anything/anyone. Am feeling pressure as have to hand deposit in tomorrow to private school. YIKES!!