Having been the teacher who organised and accompanied French exchanges, I know how fraught they can be.
Why on earth didn't the host parents get in touch with the teachers? The girl would have been accompanied back to England immediately, at her parents' expense.
We used to require parents to sign an undertskibg that they would reimburse the school if their child "brought the enchange into disrepute" and had to be brought home.. It did happen once on my watch. One of our pupils had been out all night and the host parents, correctly, declined to be responsible for his/her safety. The child was taken home the next day.