Asking What would you need to score to get a C grade? isn't a very helpful question really, as it depends on how hard the test is!
e.g. Maths GCSE, there are 2 tiers of papers, Higher and Foundation. To get a C grade you would need to get most of a Foundation paper correct, but maybe only 30(?)% of a Higher paper, because the questions are much harder.
My DD1 is about to enter y11. It seems to me that the 'pass' C grade doesn't look massively easy, despite all the media hype. A lot of stuff is 'different' from O levels, but e.g. in science there is a lot of content. History requires more reflection, using sources etc. than before, which was more leaning loads of facts.
However, I am a bit
about at least the requirements to get a A for maths, as I think that does seem a bit easy to reach.
Different people have different views of 'what is considered average'. you could get a consensus view on here, and it still wouldn't help you with talking to others, as apparently 90% of mumsnet children are headed for RG universities (mine excluded). People tend to 'average' within their field of view - so if they are looking at grammar schools they may well call their child average for getting As rather than A*s.
Talking national average, you probably need to know whether you are talking mean, median or mode, as different politicians no doubt use different averages depending on which point they are trying to prove.