GCSE French is completely different to AS. I have a friend who went from an E in mocks to a B in GCSE in Higher tier.
It can be hard enough to work with material to get a good grade, without having to 'teach' it yourself. That's why teachers are trained in teaching. I think you should write it down because it could have had an effect.
I did well at French GCSE so was 'able' to do it at AS but in reality struggled because despite hardwork, I wasn't up to it as the skills leap was too big. However, like the OP, I had the added problem where the class teaching was not in depth enough for me- save one other person, the other 6 people were literally either French nationals or had lived in France and so were almost fluent. In normal situations, ie with most normal, A*/A people it would have been fine, but with the fluency it made it too dificult and I just couldn't salvage a good grade, which was gutting for my confidence as I'd never done 'badly' before. Teaching reflected the higher group- the others knew everything! and the fluent people were often allowed to 'control' the class by just chatting away in English a lot of the time when we should have been learning from the whiteboard, which was fine for them...
The other girl and I tried so hard, but I got D, she got E. I was 'predicted B' and told I'd got that in my oral, but my marks were, in every module, low D, even though I revised so hard and didn't screw up anything. Despite GCSE top grades I admit we just weren't cut out for the course?, and the poor teaching (no extra help for us, the teachers just decided they'd get 6 out of 8 As, so why bother with people who need it) amplified this. With adaequate teaching we could have done better, but I think I was only told I could achieve a B so I wouldn't lose confidence as the rest of the class was so much better.
My point: On my own I got a D as but with proper teaching could have done better. I didn't have the ability to 'teach myself' to get a better grade like many people can, despite ages spent learning verbs and new tenses. With the OP it's much worse- she should def. mention the circumstances, as she really suffered during GCSE, missing vital teaching.