Does MG fantasy require a significant male character? If no males appear until half way through the story, is the author hurting his/her chances at publication, even if the story is aimed at girls?
I had to think about this for a while, but in the end I concluded that, no, a book does not have to have major male characters. Lots of successful books do not have any major male characters and vice versa. One of the ones that springs instantly to mind is Lord of the Rings and its complete lack of major female characters. There are also fairy-centric midgrade girls books that don't have boys. I can't think of their titles, but one of them had a little fairy that wore jeans. I can't remember much else about it, but I don't think there were any boys in it.
My advice is to write your story the way you see it. If that means that at this point it has no major male figures, then it has no major male figures. You might find in rewrites that this changes, but then again, it might not. A good book will find its market. The sex of the characters is usually a secondary factor.
That being said, I'm a little leery of the whole "no males in the first half of the story." Even if they are not major characters, most girls have interaction with males in someway. So, unless the book is set in a nunnery with no priests, a harem with no eunuchs, a boarding school with no male students or teachers, or in an Amazon-type world where males are not permitted, or some other similar circumstance, I would think twice about having a novel with no males, or mention of males for half a book. It's unrealistic in a way that's hard to suspend disbelief.