To get to the root of the problem, it might be worth seeing if there's a different family member she is willing to talk to, or if it's just because she doesn't like her options she could repeat year 1 with different options, depending how supportive the sixth form is.
If she is put on enforced study leave, and you have a good relationship normally, it might be worth drawing up a revision timetable together. Start with her deciding how long each day she wants to study for and what time she works best (don't interfere too much or she could rebel more - if she decides she wants to work late at night that's her choice, if she's at home all the time she can shift her sleep cycle anyway - just make sure she will do something each day). Then break each subject down into topics (the syllabus will be useful for this), work out how long each topic will take to study and distribute the blocks throughout the year using phased repetition. Suggest that she thinks about how she learns best and how she can use that knowledge to learn and revise effectively in less time.
By about April, she should be heading into full revision mode for exams.
As you say that the change in behaviour occurred recently, I'd guess there is an underlying issue.This could be a teacher, a friend/boyfriend, she could be stressed, the school might not be letting her study in the way she finds best etc. To reiterate, see if she'll talk to a different family member/counselor.