These women should never have put their child in that school to begin with. The school is not required to abandon its standards to accommodate these perverts. The school's rule helps assure that the parents of its students are in sync with the mission, and that the children in the school won't be undermining it with encouragement from their parents.
The mother and her live-in are entirely to blame for this mess. They are the only ones "using" the children, by making them props in their little pseudo-family.
This just occurred to me. Apparently the younger sister went to the same school for three years with no fuss. Now, this girl gets expelled after the speaking to the crowd incident.
My supposition:
Perhaps the school knew of the girl's mother and her live in homosexual partner, but decided to allow the girl, and then the other girl, to attend because it was not public. Then, the older girl made it public, and they acted.
That puts the school in even a better light. It would be educational to know all the facts.
You're absolutely right. That school has every right to only offer a Christian education to the children of the right kind of parents.