"We believe that unless Sunnie as well as her family clearly understand that God has made her female and her dress and behavior need to follow suit with her God-ordained identity...."
It seems the school has more concern with the child's behavior than her dress/appearance. The Bible passages referenced in the letter deal with sexual immorality. Maybe she displayed homosexual behavior in some form, or maybe made another girl uncomfortable, etc. The letter doesn't say.
It's a private Christian school. Why can't they deny admission if they feel an applicant doesn't meet their published requirements? Seems there's more to the story.
If the actions are based solely on a tomboy appearance, then there shouldn't be any female teachers or students in the school that has any type of short hair style. Because that is not Biblical either. Wouldn't look good for them to be hypocritical.
They've said there's more to the story, and I don't doubt that.
I just can't figure out why, if there was some specific, disrupting or inappropriate behavior, they wouldn't have mentioned that. That's a very simple issue to bring up with the grandparents. If you are going to go there with this, I'd think you would mention some specific behaviors.
Somebody mentioned maybe protecting the girl by not mentioning a particular disturbing behavior, but this letter was to the parents. It wasn't meant for the public, and it was always the parents' choice to share it.
No reason to hold back, and it would be irresponsible not to share something that's legitimately a problem with the grandparents.