Americans coming to the UK with teens - help!

Part of my job is to help people relocate her and many have kids and many are American.
Most of my clients use Private schools but most of them do drop back a year, the last one who didn't came from a very academic Private school in The US and coped fine. I did have one who had to join the correct year as there was no space in the one below and they did struggle
As for State schools your friends will need a UK address first and will then ahve to find a school with spaces, some State schools will allow a child to be in a different year but most won't but in any case it should be house first then schools.

This is the situation with secondaries in much of the country. I believe Cambridgeshire and Oxfordshire are particularly difficult:

https://www.bbc.co.uk/news/uk-england-cambridgeshire-68665716

Okay, so they'll be going into Y9 and Y10? They need to find a place to live asap, as they won't be able to apply to the local authority for school places until they have an address within that school's catchment area. So they get their address, they apply and then they either get told where the LA has spaces, or if they move to one of the over subscribed areas of the country then they go on a waiting list and their DC might not get school places straight away. There are some year groups nationwide that are full in many LA areas, so they should call up the education depts of any LAs they're thinking of moving to and check.

As for the GCSE options, the one going in Y9 will be okay, because s/he will be able to pick options next spring, the older DC may be more constrained, depending on whether some courses are already full, but the school will try and accommodate their wishes, as much as possible. If they've been in the US education system up to now though they may struggle with gaps in their knowledge, as the two countries have quite different education systems. Personally, I wouldn't do this to my kids, but people do ...

IMO the best strategy would be to:
Pick an area with overall good secondary schools. Hampshire is often mentioned here, but you could start a thread asking for areas with good secondary schools / not oversubscribed.

Arrive in July before the end of the school term. get an address close to a favoured school or a cluster of good / ok schools .

Contact the LA and get in the system for in year places in September. Get on the waiting list for every ok school they can travel to.

Start thinking about what GCSE options they aim to take. Do research into how the curriculum works in England . E.g do they want to do triple science (3 GCSEs) or Combined (all 3 subjects but combined to equal 2 GCSEs) .

This is a good start into GCSE choices, but individual schools will have different language offers etc. https://www.bbc.co.uk/bitesize/articles/zrjh92p#zjr8s82

Keep fingers crossed!

I'd move earlier than the end of the US school year (Easter holidays maybe) to try and secure a place for September, they may even slot into spaces that are already open before the end of the school year. Everyone who moves seems to do it at the end of a year or in summer and expects a spot for September, but if they are already in just for the end of this school year it'd be better.
We moved from US, after the year had started in US but in the first week of Sept in UK, we knew the local school had room as family had enquired with the head and she was expecting us.

If they have the money (I realise this is a big if), the best thing might be to go private for years 9/10/11. It will be easier to find a private school place and the boys would be well supported with the transition. They could then apply to sixth form colleges and revert to state for years 12/13.

I would advise them to do their homework before deciding which area to move to. I live in Bristol and it is very very difficult to get an in-year place in any secondary school, let alone one of the better ones. Most schools (here at least) admit on geographical proximity to the school, so if you could find a house right beside the school you would go straight to the top of the waiting list. However, there is a huge demand for rental properties, so finding any house, let alone one that is beside your preferred school, is a challenge. It's also worth saying that it is very unusual for children to move school in year 9/10 for the reasons mentioned above, so the likelihood of places coming up is quite small.

They will need to pick an area carefully and watch out for schools that start GCSEs in Y9 (there are more than a few).

Also look at doing some tutoring this year to catch up to where they would need to be in core subjects.

Mainly they need to decide where they are planning to move to and be aware that these are very high birth years, some councils have no places available for state schools in those years and that they realise it's not like the US where you are guaranteed a place in your local school - they will be taking whatever is available, that can be up to 75 minutes travel each way, and may not be the same school for both children.

Ideally they would start school in September, there could be complications with options etc.
However, state school in my experience will do their best to accommodate special cases. I have known a student arrive at the beginning of year 11 and still come out with some GCSEs.
It will be tough so late in their school career as the curriculums are so different. (They could for example be both ahead and behind in Maths.)
I would suggest making the move ASAP, and living where they need to for Jobs or family.

It might be easier to do so as soon as US school finishes and get places before UK schools finish in July. Most schools formally start gcse work in year 10 and choice of subjects beyond compulsory core of English Language and Literature, Maths and Science will vary, both in number and range. Usually a language, humanity (History, Geography,etc) plus a choice or two.

They can't apply for state schools until they have a UK address.

Initially they should contact the LA regarding school admissions

However it will do no harm whatsoever to proactively contact schools - the LA website will have a list of schools. They should ask to speak to the admissions officer & get info about how likely it is they will receive a place.

If once they have moved, they can't find a place at a preferred school, the LA will find one for them. It may not be particularly local or particularly good (schools with places in late secondary tend not to be popular).

What age will they each be at end of August? They would make InYear applications once they have an address, some LA will process in advance of arrival but they will need to be in a position to take up places within weeks. If a school has a space in the right year with no waiting list it is relatively straightforward but quite possible they will be at different schools.