What does she do when you leave her? Does she just miss school?
The stomping around, looking for a hoodie may be her way of avoiding school without admitting to herself or you that she just doesn't want to go.
I'm guessing that repeating year 12 was a blow to her confidence, however she's trying to mask that. She'll be embarrassed (her friends now see her repeating year 12) and a bit scared (she's failed once, it felt awful, what's to stop her failing again?).
Obviously, a lot of the above is illogical: if she gets herself together, there's no reason not to do better this time; her friends probably want her to do well, as do you. But it's hard to be logical when you are a. young and b. in the middle of something.
Realistically, you have no choice but to leave her if she is going to make you all late. I'd try leaving her for a few days and if it becomes clear that that isn't working (getting her to hurry up), sit down with her and ask if she really wants to go back to school at all, what her alternatives are.
If she says she does want to go back to school, that gives you something to work with - her sense of commitment and control and ... leverage. really.
You can get her to come up with solutions (clothes out the night before, getting up earlier).
If she says she doesn't want to go back, you can look at doing A levels on-line.
You have my sympathy. It's horrible to watch them making such a disaster of things.