Well, if you are planning for her to go the the State option, you'll have to hope she doesn't get the requirement for her fee paying school, as if she does, you will owe the fees as per the terms and conditions. And no a charity won't want to spend its money on a term of fees where no education is received.
You might find that if the school knows she has been funded by a charity, they waive the notice period, especially if she hasn't done so well.
Worth bearing in mind that often the course requirements are there for a good reasons - for example a maths requirement for A Level science - evidence suggests A level science outcomes with low starting GCSE in maths are pretty poor - just worth bearing in mind.
Anyway, on one level, until the results comes out, you won't know how she's done or what the next steps are. Perhaps things will be better than expected and she will stay where she is. If you want clarity about the fees, get in touch with the Head or Bursar and ask about these scenarios;
- DD fails to meet school entry requirement to 6th form
-DD fails to meet requirement for courses she would like to study
Make clear at start that you are worried about her having to leave and you being charged a term if fees and that you receive help (state %) from charity (name them).
Also be clear in your own mind if she will stay if she DOES meet the entry requirements or if you are planning to move her anyway.