I think Pugsandseals may have a bit of a point if she is driving at cognitive bias playing a part in identifying which child can go so far. I will attempt to describe below:
It is true that the NC system essentially divides the cohort up into high, middle and low attainers/ability groups - although a good school will expect all groups to make as much progress as possible it is true to say that the high ability group will be expected to get higher academic results than the low ability group.
Our school is extremely large and 'all through' from nursery to sixth form. It is extremely well regarded with outstanding teaching and outstanding Ofsted reports in both Infant, Junior and Senior departments. I can't think of a parent who wouldn't recommend our school for many good reasons. In our school the high, middle and low ability children are essentially identified by the end of KS1 - if not before. CATS/PIPS type tests and SATS, both tests and ongoing teacher assessments are used to identify the high, middle and low ability. By the end of KSI impressions have generally been formed of the children some are 'quick' and some are 'slow' some are obliging and some are more challenging etc. We would all make these judgements as it is only human to do so. All the interactions are reduced to a manageable number of characteristics. Impressions are so formed and once a child is seen as bright that judgement is not usually rescinded - especially when you have the data to back up your judgement.
At the end of KS1 predictions are made for the end of KS2. Our school believe in setting for Maths by ability. One of mine was barely 6 when they were set for Maths. Because they were a 2a at the end of KS1 they were placed in a set with other children who were also working at a level 2a. The expectations and trajectory for them was not the same as those working at 3c or 3b etc at this time. There were 8 sets and each set worked at a slightly different pace from top to bottom.
I went to a talk about the maths setting and was told that those who were predicted a low level 4 at the end of KS2 might expect to use different calculation methods etc. So judgements and expectations were alive and kicking. By the end of Y3 my child was working at a level 3b - those in the 3 maths sets above were working at a faster pace with higher expectations and I imagine the highest ability group were then around a 4b or even higher by the end of Y3.
In Y4 my child was working at a 3a in the first term and I was told that they were predicted a 5c by the end of Y6 if all was well and good - very good for a child that was a 2a at the end of KS1. I was also told that they believed strongly in setting by ability again and that as most were expected to be a 4b by the end of Year 6 I should be delighted by that level of progress.
I asked the question why not a level 6 at the end of Y6 if ability had developed and warranted it? No reply was given. It's clear my child wasn't marked out as level 6 material.
Does any of the above matter? The jury is out for me. I think some can be penalised by the system but it's those at the margins. They do move children between sets but from what I can tell you have to be truly exceptional to warrant a move as it's tricky to do so administratively and as they rarely move children down (perhaps as CATS tests etc show they have the requisite ability to stay in a higher set?) there isn't always the space.
You can see that those who started at 3c on entry to KS2 in a system like this may have been advantaged? They would definitely be in set 1 or 2 and working with high expectations in mind. KS2 results influence GCSE results and so on and so forth. Those that were clued up in our school got on the Kumon bandwagon our used tutoring to get children up to a 3c where they were at the margins. Still not sure where I sit with it all? If intellect is fixed then I expect they are all pretty much where they should be and you can't have a system that works for everyone perfectly? Surely expectations and predictions are made in every school?