My son birth is 2008, he had done a couple of years outside the country.
He will be in year 6 in September.

1) Is he late to prepare and register for the exam?
2) Does he need to apply for each school separately?
3) Whats best way to get started?

What area are you in?
If you let us know, I can transfer your post to the relevant section.
The 11+ is not standard across the country.

London area, Thanks
Idea is to get into grammer school

There is no 'London 11+', though - there are grammar school in Barnet, Kingston, Sutton, Redbridge, Enfield (forget those last two, unless you live within their respective catchment areas, really - although for Latymer in Enfield there is a short window of opportunity for moving into catchment post-results). I may have missed of or two, but essentially they are all in outer London boroughs, so not very convenient to get to some if you live somewhere convenient for another.

(However, if you live in somewhere like Hounslow, after allocations day in March there is usually someone looking for a taxi share / car pool to one or other of the ones which don't have catchment areas).

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Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.Groucho Marx

If you look at this section https://www.elevenplusexams.co.uk/schools/regions
It gives you links to the different areas & requirements. If you have been out of the country for a little while, I would gently suggest that a grammar school is not the be all & end all of education, you may have excellent local schools, so have a look at those too & don't dismiss them just because they aren't grammar schools. Grammar schools follow the same curriculum & sit the same exams as all state schools. If the comprehensive schools near you are poor, I quite understand, but it can be counterproductive to ship your child around the edges of London to sit the various grammar school exams simply to get a place at any grammar school. They have good results, but that's because they cherry pick their students. Also choose a school that you think would suit your son.

London is a very large place.

You need to give us some idea of where you would live and we can give you some idea of what might be possible for you.

Motivation is the key. I have worked with students who have started in July and done really well because they are very focused. Others start early and go off the boil. DG

I am close to Alperton area, my confusion is because he will go in year 6 in September,
1) Has he lost the chances of entrance exam, is it too late to fill school forms for the exam?
2) Do I need to fill each grammar school form separately?
3) his date of birth is March 2008, he has studied in London till year 3, the last two years outside the UK.
Will he be eligible for grammar school

Which schools were you considering?

I don’t know the area at all so it is difficult to give you advice.
The 11+ is not standard at all and different local authorities have different rules.

On the grey bar at the top of the page there is a schools bit, if you click on that the drop down menu there is a lot of additional information.

Is your child at a UK primary school at the moment or won’t he start until Sept?

1) almost certainly not - you need to look at the website of each school for its admissions policy and to check the closing date for registration.

2) Yes (Unless the school is part of a consortium, in which case, sitting one exam gives access to all)

3) If he is eligible to be educated at public expense, so to speak, then he is eligible to sit the entrance exams for state grammar schools and to attend one if he does well enough, applies for a place and is successful in being allocated one (if that's what you mean?).

Possibly the only grammar school you can 'reasonably' consider from Alperton would be Queen Elizabeth in Barnet - only two bus rides and less than an hour and a half away and open to any boy from anywhere as they admit on score in the exam only.

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Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.Groucho Marx

There are good schools around Alperton. I would save the heartache of grammar school exams & at least consider those. Remember, the results may not look as impressive overall, but that is because they take all children. Find out how the high achieving children do at those schools.