Take child out of private school? whilst one remains

[sef23]

Have two children in a private school in wales. One is starting to excel but is year 9, other who started last year and is now in year 6 is struggling with the work. I would ordinarily put her back in the primary she is from, but its a low performing welsh language primary and she hasnt done welsh for a year and a half. She was put in Private for smaller class sizes to help her transition to English but the support hasn't been there, eg teacher said shes about 3 years behind in Maths and her writing is terrible. Im failing her is an understatement shes also losing her external circle of social friends outside of school as they are moving on, the private school kids do not socialise outside of school as they live all over the place. I want both to be the same chance - son also hates it but is doing good academically as he also wants to be with his old friends but he has a robust social life outside of school so hes doing ok mentally. This decision is so important, do i put her back or keep her in? Welsh school teachers said she should catch up and then go to her local comprehensive.

My friends did that several times over. Both in private primary, one was doing well academically and went on to private secondary, the younger one wasn't fussed about studies (they have a creative streak) and was put into a state secondary where they could fill their creative potential. Fast forward a few years, they both had terrible time in their respective schools so the younger one went to a private (they had specific needs that couldn't be fulfilled by state schools in the area) and the older one chose a state school where they could have better pastoral care going into their A levels. Both kids are now happy where they are and have no gripe about who is doing what and who got more years in a private school.
I think it is more important where you child feels supported rather than private vs state education.