But, Captain, I have also taught students who have been 'dragged' through a curriculum that they were clearly not ready for - schools that have mixed ability maths classes or parents who insisted their child must be taught the whole syllabus, etc., and in several cases, that failed the children as much as the opposite. They did not understand that level work, but there also wasn't the time to spend going over the real basics that they did need, so in the end they came out with nothing. They would have been better not doing some of the highest level topics, and instead really solidifying things like fractions.
On the other hand, those mixed classes where everyone gets taught everything so that they have the potential to achieve higher, has worked for some, particularly those who didn't shine initially but got to grips with it later (either maturing later, or just starting to work hard at that point).
And I've also had students failed by the classes that don't teach everything, because in some cases, those children could have succeeded in the topics with a bit of pushing and earlier hard work. they have come for extra lessons near the end, having finally realised they need maths, or want to do it, or things have clicked or whatever, but they are stuck in a set that only teaches them up to B grade work. This tends to be quite a last minute thing though, as schools generally move them up sooner if it seems like it's going to be possible - and in the case of the children I know, they fully accepted that it was their responsibility for not having worked hard sooner, rather than the school holding them back.
So it is quite hard to know how to best meet the needs of all.
Even with a poor teacher, though, a real mathematician should find the work fairly straightforward because it just fits together and makes sense. There should be no reason not to be getting all the basics right. Specific topics for A or A grades, like sine rule etc, are different, and if a child hasn't been taught those then that could be an explanation for not getting the higher grades, but really if someone is not finding maths almost trivially easy through GCSE, they will struggle a lot with A-level. It doesn't mean getting an A or A at gCSE, as some very able mathematicians don't - through being careless, finding it almost too easy to bother with, not showing enough work, and other exam-strategy problems (that do need to be worked on), but if they aren't finding the work/concepts itself very easy, then A-level will be a challenge. In particular, the unstructured, multi-step problems at the end of GCSE papers, where they have to incorporate ideas from a couple of different topics and work out what is needed, are things that they should enjoy the challenge of.