I believe in the concept of learnable intelligence and if a group of children are treated as if they are intelligent over time they become so to a degree (the reverse is also true). There is heaps of research out there to back this up. IQ is multi faceted and involves habits of mind which can be trained to some extent. Confidence also breeds increased confidence and success breeds success.
The school system measures and praises a certain type of intelligence. Intelligence tests measure verbal and logical reasoning ability. There are many other types of intelligence that the tests are not designed to measure. Potential can also flower later.
All those in my primary who appeared bright early on, good 'secretarial skills' - great joining, good finger spacing and no work spoilt due to poor presentation had involved parents who knew how to play the system, well groomed and keen etc, these kids tended to pass the 11 plus or were put into the top stream at our local comp. In my opinion others, just as bright, were treated as less so and this became a self fulfilling prophecy going forward for far too many.
Parents and even teachers in some cases wishing to protect themselves or their children/pupils from embarrassment say things like 'John will never be any good at maths, he's reached his ability ceiling, its not his thing. He's better at football' and every time the phrase is repeated it reinforces the child's mental paralysis. Innocently and unintentionally the doors of possibility are screwed up.
A frightening proportion that start out in the top ability tables/sets seem to remain there going forward. Those that work their way up from the bottom to the top are there but most would agree they are in the minority especially as you get to about year 3 plus. How many threads on Mumsnet mention the reception teacher who can predict the 11 plus successes with 90% accuracy. How depressing. A late developer threatens the authority of those who wish to put children into watertight categories, later on he/she calls into question the credibility of academic qualifications and casts doubt on the school's judgement. In short late developers are subversive.
I know children who have failed the 11 plus - in the Bucks area- appealed and gone to to get brilliant A'level results.
I guess when resources are scarce it boils down to who deserves the best education?