CIS : languages

I attended the information session last week and there was a short presentation given by the head boy and head girl. The head boy said he started in CIS since reception, but i was a bit surprised that his english, while very fluent, is not native at all and retain quite a bit of HK accent, whilst his mandarin is simply appauling. The head girl instead said she was with CIS for only 3 girls and both her english and mandarin are quite native. Anyone knows where she is from Also, is langugage environmnet in CIS not enough, or simply because boys aren't as good as girls in second languages?

Hope to know more from CIS parents, and from parents whose kids had spent quite some years in IS.