Because the school dd would have got is awful in pretty much every way. And I know it's awful for the kids that had to go there. Most people avoid it. The school that she could hope for, but be unlikely to get, is good in lots of ways but still offers nothing to able dc. And I can't afford to move into catchment for a better state school, nor have I had the time to suck up to a church.
Private was because I wanted her to attend a school that offered an academic curriculum and suitable challenge. Possible because she got a small scholarship and a large bursary.
If she attended the dire school, I can only guess. She still socialises with local dc and lives here so no change in that regard.
Educationally, she'd be under the impression she could effortlessly be the best in school and that formal education was boring and easy. I'd have put her in for a maths gcse and maybe another to either appeal for a school that offers more than the legal minimum of academic subjects, or at the least to avoid the boredom of being taught maths she did years ago by a teacher who has no qualifications in the subject. I'd be homeschooling in the evenings, or maybe I'd have arranged pt school and dd studying while I was at work.
I'm also sure she'd be getting a reputation for being a trouble maker, because she's disgusted by the way some of her friends are treated by the school, and I'm under no illusions she would keep quiet in school.
Only possible gain if she'd gone there would be that a few less confident friends who have been/ are being bullied would probably not have been if dd had been there as she's confident. We've already had fall outs on social media and in one case a physical fight with one local bully, started by the bully and ended by dd before anything was hurt other than the bullies pride. And her friend with sn wouldn't have been managed out so easily because dd would have defended her from other dc and pissed off teachers asking in every lesson why her friends 1-1 was with y11s.
Hardly what I want from my child's education.
If the comprehensive system itself was fair and dc round here could attend the type so many mumsnetters consider the norm, I wouldn't have investigated private. Or if we had a grammar school in feasible travel distance. However now she's there I realise what she has is a big advantage on a good comprehensive, and given her other disadvantages in life, it evens things up between her and the many privileged dc at good state schools.