I Cor 5: 1ff- 1It is actually reported that there is sexual immorality among you, and of a kind that does not occur even among pagans: A man has his father's wife. 2And you are proud! Shouldn't you rather have been filled with grief and have put out of your fellowship the man who did this? 3Even though I am not physically present, I am with you in spirit. And I have already passed judgment on the one who did this, just as if I were present. 4When you are assembled in the name of our Lord Jesus and I am with you in spirit, and the power of our Lord Jesus is present, 5hand this man over to Satan, so that the sinful nature[a] may be destroyed and his spirit saved on the day of the Lord...
...9I have written you in my letter not to associate with sexually immoral people 10not at all meaning the people of this world who are immoral, or the greedy and swindlers, or idolaters. In that case you would have to leave this world. 11But now I am writing you that you must not associate with anyone who calls himself a brother but is sexually immoral or greedy, an idolater or a slanderer, a drunkard or a swindler. With such a man do not even eat.
12What business is it of mine to judge those outside the church? Are you not to judge those inside? 13God will judge those outside. "Expel the wicked man from among you."[b]
New International Version (c) Zondervan.
I think you missed my point. That's fine. We don't have to agree, okay?
So a person can become wicked because of a choice someone else makes that they are powerless to stop?
If the school wants to have a code of conduct for it's students and require them to agree to certain teachings, fine, but to punish a kid for something they are not accountable for is wrong and stupid. This kid will have enough troubles in life without so called Christians pushing her away from the thing that would give her peace.
"...I have written you in my letter not to associate with sexually immoral people not at all meaning the people of this world who are immoral, or the greedy and swindlers, or idolaters. In that case you would have to leave this world."
Expelling the immoral applies to avowed Christians, NOT everyone, and certainly not people who are merely RELATED to the immoral people. Jesus' ancestors included prostitutes & incestuous relationships, after all.