Sure there is.
1 Thessalonians 5:22 "Abstain from all appearance of evil."
The school may feel that, by allowing children of parents who violate the 'morals clause' of the schooling agreement to keep attending, the appearance of condoning such behavior may be perceived.
Whether you agree with that view or not is another story. But obviously, these rules are in place for a reason. They have probably experienced repercussions in the past for allowing a child to stay whose parents had violated the 'morals clause'. Or it may even have occurred before the morals clause was in the agreement and that's why it was added.
The simple fact is that the school has made an economic decision. They fear that parents will begin pulling their children from the school if they allow this girl to stay. There was a similar situation recently in which a supposed "Christian" school ejected the child of a stripper. These are not attempts to keep evil influences from the school, these are attempts to cow-tow and knee-jerk to people who can't seem to mind their own business. If the school is doing its job, rightly in the sight of God, the children will see the error of their parents' ways and live Godly lives. It is not the job of the school to correct the parents, that's for the parents to do themselves. The school's job is to provide a way for the word of God to come to the children and let Jesus do the rest. And believe me, given half a chance, he always comes through.