@AskingForAFriend10 no for sure you don’t sound aggressive. And I am sure your DS will improve and do well – as I said before, all the best to him. Obviously things can be different from the overall stats in a specific case.
I don’t agree tho that setting a minimum standard for moving from GCSE to A levels is a barrier to social mobility. I am all in favour of social mobility, but allowing students to do badly at A level is not the way to promote it. And I also don’t agree that mentioning it “once per thread” is enough. As @TeenToTwenties says in an excellent post, every year on MN there are parents whose DC have taken on A levels that are beyond them – DC who would probably have been better suited to a different style of course, but which they have apparently not considered. Btecs can lead to a uni place for sure and should not be looked down on if they suit a YP better.
I was a teacher in school and am also a parent of three DC who have been through this, with their friends; I have seen and heard of numerous examples, in my subject (MFL) and others, especially maths, where unfortunately a student was allowed to continue to A level when it would have been better if they hadn’t, and ended up dropping out or gaining a much lower grade than hoped.
My specific subjects are French and German. There is no scope in the complex and challenging A level course to reteach French verbs or German adjective agreement; that knowledge is assumed at the start of the course. If a candidate were working at a grade 3 at GCSE (equivalent to the D grade you mention), then they clearly would not have the knowledge needed. A levels have been reformed in recent-ish years and are very challenging and a big step up from GCSEs, hence schools and colleges being keen to ensure that the necessary ability, skills and knowledge are there at the start.
Yes if a student as @TeenToTwenties says had had some dramatic reason why they got a 6, not the expected 7 or 8 (major trauma in the middle of their exams for example) I am sure a school would bear that in mind. And yes, gaining an 8 or 9 at GCSE in no way guarantees an A or Astar at A level, I know that well. But it’s a better start with more chance of a positive outcome. A levels are not for everyone and there are other routes that may be more productive for a candidate with lower GCSE grades.
ETA: in no way did I intend to talk down to @AskingForAFriend10 or their hopes for their DC and I apologise that it may have read that way. I just wanted to flag that the stats are there and they are best considered. If a DC improves from a grade 5 to a grade 7 and then gains an 8 in the exam, happy days. Just if a student (any student, not the PP's DC) ends up with a 6 in maths then they may want to reconsider maths A level.
But I hope I have clarified that I am certainly not saying that their DC will not make the grade, and I agree they should pursue the sixth form (if it is part of a school - that's not clear tbh) and make sure it is not breaking the rules. But still - have a plan B as well.