OP maybe I’m being naive, but why would there be a quota for top grades? Or do you mean that the standard for A grade might be set higher if there are native speakers?*
My understanding is that yes, there is a quota for the top grades. Only a certain percentage get an A, a different percentage an A etc. It is not so fixed across subjects, they apparently look at attainment at GCSE across the board to get a feel for how many "top" students do a particular subject (otherwise anybody doing FM would be very disadvantaged, as everybody doing FM is likely to be a very good mathematician), but they somehow then use these statistics to allocate a certain percentage (within a tolerance) who get an A*, A etc. on each subject (at least those where there are around 500 students a year or more).
If, say, because of the drop off in languages (both at GCSE and particularly at A level), native speakers make up a significant cohort, then the non native speakers will be facing an additional challenge, as the standards will be higher, and they won't be starting from a level playing field. Not that students do ever, but - say take the other subjects my DD wants to do - biology and maths. The number of people who learnt A level biology or maths content because their parents talked to them about it when they were a small child or lower down in school is neglible. But not quite so true for A Level MFL.
@TheresACrackInEverything - yes we are very worried that the school will not offer French. They have in the past if there were 2-3, but we don't know if there is going to even be that. The GCSE class she is one of 6, but I gather one other might be interested. We shall have to see. Some people join the school at sixth form too and they have had French takes from the new joiners in the past, apparently.
She has been sending me links to places like interhigh - and we have wondered whether she should consider taking the French online (if the school won't offer it). I haven't dared ask the school if they would be happy with her taking two A levels with them and one oneline, and don't know if online is a good idea.
The other option would be to take her other MFL for A level instead of the French (she doesn't want to carry on doing both, although she is good at both, the two of them are her strongest subjects, but she is thinking of studying more in the biology area - ie not currently thinking about languages for university- but of course that might change). There are going to be more students for the other MFL in her school (which is not one of your classics, more of a community language, although she is not a native speaker of that either - and there are quite a few native speakers in her school in that, although most of them took the GCSE early, in Year 10, the ones doing it at Year 11 are generally the non native speakers). She prefers the French though. Feels there is much more interesting accessible material in it.