These "parents" are unrepentant lesbians. By allowing the girl to attend the school, the "parents" would be allowed at school functions which would give the impression of approval of their lifestyle. What would you do? Ban the parents? You couldn't.
There were 2 new students, girls, who had a "relationship" at my daughters school. They were told to give it up or leave the school. They left. There can be no compromise on the Bible and Christians have to take a stand. What do you think would have been said if the school had asked the girls "parents" to repent as our school did with the 2 students? The "parents" would have had a cow.
Who knows? I would certainly be willing to risk asking. I don't think that anyone is going to hell for loving another person, but the expression of homosexuality is clearly against the commands of God. If you part company, you part company, cow or not. Why not just ask? Why not talk about it? If you are willing to ask the question, "Are you gay?" then why not posit, "This is a Christian school, meaning that the covenant you signed indicated that you are a Christ-follower. Do you know what that is all about?" and proceed from there.