Secondary school advice for someone who knows nothing about the British system

[EscapeInThePark]

So I am French and have no idea how the secondary school system really works here.
My oldest dc is in Y5 so I am starting to look at school around. But I would like to know what it is that I really need to check/know.

So far I have gathered that not all schools provide the same options at GCSE so that might make a difference (ds is doing well in maths and could potentially do further in maths/science).
There is also the issue of how well is the school doing. Is being in a better rated school really makes a huge difference later on (ie GCSE, A levels and then applying to Uni)?
What I am not so sure is if you have to attend the same secondary for GCSE and A levels or is it quite common for children to move at that stage?

Any comments would be really welcome.

OP, don't worry too much about whether schools are 'academies' or not...the Government is encouraging all schools to become them again from the school's point of view it means a little extra money my school is a selective grammar and we are an academy, although we do not say so (you don't have to change the name of the school). In the early days academies were re-branded versions of schools that had previously had problems, generally then put into nice new buildings with a new name. But now it doesn't mean very much.

Glad the Russell Group leaflet was of some use. I thought it was quite brave of them to put in writing what most people suspected anyway.

Good luck with your school search.