I'm still a bit confused. I've looked at the new appeals code and in the bit on 6th forms it says 'the panel must not make its own assessment of a child's ability
What this means is that the panel cannot, for example, set its own tests to assess ability, or 'interview' the pupil. What they can
do is to look at any alternative evidence you provide, e.g. previous good exam results/SATs levels, good school reports.
Am I therefore trying to prove that the GCSE results were not a reasonable representation of my daughter's ability?
Are the school just going to say that it's a black and white situation and that she didn't get the grades and that's that?
That will and ought to be their case. Admission arrangements are meant to be 'black and white'.
Will the Panel still have to look at whether the admission arrangements of the first stage?
Yes, and it's quite likely they will find that everything is in order. (It would be a matter for concern if not.)
You then counter this by asking the panel to consider other evidence of academic suitability, and by pointing to any extenuating circumstances.
can I still bring up extenuating circumstances?
If you think they led to an underperformance, then yes - ideally backed up by evidence.
You may of course be asked "Has she now put the extenuating circumstances behind her, and will she be able to cope with the pressures of the 6th form?
It's perfectly possible to win an appeal like this, but it all depends on the evidence.