Sorry this is a bit long, but I’m trying to explain how work is moderated.
It is standard practice to downgrade all candidates from a centre if the moderator perceives there to be significant inaccuracy in the original mark. This document explains the process
how moderation works
but there is a summary here:
I think the third option has been applied in OP’s case.
There are four possible outcomes of the moderation process:
(i) There is no difference between the between the centre’s marks and the moderator’s marks for the sample - so the centre’s marks are accepted for all candidates
(ii) There are only small differences between the centre’s marks and the moderator’s marks for the sampled candidates - so the centre’s marks are accepted for all candidates
(iii)The differences are greater but the moderator generally agrees with the centre’s rank ordering of the candidates so the technique described in the document above is used to adjust the marks of all candidates.
(iv)There is significant disagreement between the moderator and the centre’s rank ordering of the candidates so further work is looked at. In extreme cases this may lead to the moderator’s marks being imposed for all candidates at the centre.
From this year candidates HAD to be given 10 days notice of their marks before they were submitted to the board, in order to allow them to appeal the marks if they thought they were wrong. This has made it harder to appeal but you can try.
I find it unlikely that marks will have been reduced because of “too many high grades being awarded” as the Moderator’s marks will have been submitted before grade boundaries were decided.
OP please email the Headteacher, as this is time critical. Ask DD if she was given notice of her marks and made aware of her right to appeal before submission.