Don't agree with all Pugs was saying at all but at least in our school:
EYFS scores inform informal setting in KS1
KS1 SATS and continuous assessment decisions inform setting in KS2
Those predicted a low level 4 in KS2 SATS are expected to use different methods from others (we know who these are - generally - at the end of KS1)
The cohort is seen as HA, MA and LA (as it is generally elsewhere)?
There's a chance outliers in various sets may not reach potential (hopefully this is only a small chance)
There can be ceilings on the ability of the 'outliers' to access more advanced curriculum
KS2 scores help inform GCSE predictions
Progress is tracked from EYFS and doing well early on - through KS1 - mean academic expectations will generally be high level 3s at KS1 'need' to be level 5s at KS2 for adequate progress to have been made etc.
Not to say that things don't change and there won't be movements through ability groups etc.
Just wondering if this is what Pugs was driving at when she said - I think - something like 'it was all set in stone from EYFS'? It's not but perhaps some of the above may be what she was really driving at?