So tomorrow I have a meeting with her headmistress, do I get on my knees and beg?
Lisa, if you think it will help, do it! :-) It would be worth it if it gets your daughter a chance to work at her potential.
Take along the article on Cutting Down the Poppies to give the headmistress; that's a great one (see the articles section on this site).
And you know, about maturity, the problem with that argument is that there is rarely an ideal fit for highly gifted kids. There are parts of them that just won't fit in anywhere. If your daughter stays where she is now, it's a poor fit academically--which is damaging to her. Possibly it's a poor fit socially, too, if intellectually she can't relate to her age peers. If she's grade-skipped, yes, there may be times when she acts her age and is less mature than her grade peers. But how is that any worse than the poor fit she has right now?
I'll be thinking of you tomorrow. Best of luck!
GG