I’m also raising my children bilingually, using “one parent one language” method which worked very well for us. They might have found first couple of years in English only school hard as I never have spoken to my children in English. However, once they’ve learnt to read, their English quickly caught up and then out perform other English only children which amazed me.
According to the report like below, bilingual children have a lot of advantage, academically, socially and linguistically.
http://www.ecolebilingue.org/01_present ... tu_en.aspx
May ask why you think your child would be able to cope in a demanding Grammar School regime if you think your child’s ability of English superficial?
In my daughters Grammar School, the majority of girls now are bilingual Asian and the number is growing.
So I don’t think it’s easy to convince an appeal panel concerning the negative affects of having two languages at home.
I’m sorry if it sounds a bit harsh to you, I hope all the best for your child.