I think there are 2 different issue so or you to consider her Op;
Firstly, given her achievement level, are A Levels the right option for her and especially continuing into the u6th from this low starting point?
I think this is the key question. You might feel you've not been informed enough and she is entitled to stay, but at the end of the day, having what you're entitled to isn't always the best thing - is it the right thing for her to move into something else, regardless of the rights and wrongs?
Secondly, has the school acted wrongly? Schools are pretty careful not to just remove someone without going through the channels which are needed - so warnings to pupils, setting clear requirements to progress to the next stage, contact with parents etc. Has your DD told you fully what happened last year and passed on any communications and have you fully read the school policies and anything sent home to you?
Regardless, you need to go in for a meeting. You need to establish her performance and if continuing to the U6thbis a good idea. If she's likely to get Us or Es, she could probably do something more productive with her time. You need to be willing to consider this if that is the reality. You also need to establish what's going on with communication from school - perhaps it will turn out they totally failed to suggest to her or to you that there was a risk of not continuing.....but I'd be surprised if there was no communication.
If you go to the school with a positive 'I've come to find out and clarify what's been happening' rather than a 'I'm furious and have come to sue you' approach, you're far more useful to have a productive conversation. I can see you are shocked by this, but try to be open minded to hear about what has happened and what their recommendations are for your DD for the next stage. It might be possible for her to stay, or it might not. It might be a good idea for her to stay or it might not......stay open to the idea that just continuing because it seems easiest isn't actually always the best option and achieving little at the end of 2 years is worse than achieving little at the end of 1 year.
I'm sure that if you approach them constructively you'll be able to get to the bottom of this. And good luck to your DD too in being able to approach this in a mature and open way, looking for what is best into the next stages.