I am struck by how high quality teaching early on can influence a child's academic ability and so influence a child's performance in the 11 plus as well as more broadly. A child's self belief is also hugely important.
I don't agree with LeQueen. Those with an average IQ, if that's what you want to call, it can't 'become' Grammar school material. Grammar schools are streamed so those that grasp concepts extremely quickly will be separated out but disagree that the 'average' (whatever that means) will struggle.
What you 'believe' you can do with hard work and determination etc is far more important. I've spoken about Mindset by Carol Dweck before and notice that this book is now being recommended by schools. All should read it and buy it in my opinion.
Our local non selective prep early ALL bar about one of the children achieve level 5s at KS2 or above. Not all are natural geniuses.
Take comprehension as an example of how great teaching can make a real difference; at my school, when we were about 10 years old, this was introduced to us. We were told to read the question and the passage twice. This was where our teaching began and ended.
We had laminated cards to complete in our own time. We received no feedback apart from our score. A high score over a few weeks meant we moved to the next graded level.
A good teacher will explain that in order to do well in comprehension and get the most from it a child should read broadly. A child should perhaps aim to read most of the children's classics by the age of 11. Why not? If they get stuck they are being taught in class, on an ongoing basis, how to make some sense of the text. Hopefully their teacher or an interested adult will help them if they get stuck.
They are taught how to infer from the text, they are taught how to identify key words, how to understand genre. They are taught how to 'read' the passage in a helpful way. They are taught how to underline key words and skim read. What about the characters? How many are there? What are their traits? What might an examiner want to know?
After a good time these questions will be in their heads and become second nature. A sense of confidence will be instilled in the child. 'I can do this'.
All aspects are broken down to simple parts that most will be able to understand in time. If they have to identify an unfamiliar word they are taught to think about similar words with similar roots, beginnings etc for context. So on and so forth.
A child that is given the laminated card approach may do well but there will be plenty more with 'average' scores that could be taught to do extremely well over time. You can be taught how to 'think' and how not to have self limiting beliefs.