Why do you call college school?
Why do you need to go to college for 4 years and then another college to be a Dr, lawyer etc? And how does anyone afford 8 years college?
Re final years of secondary - it's not simple, and this is England and Wales - Scotland has a different system.
High school is 11 - 16 (can also be a grammar, secondary modern, or another name) and you used to go in to 1st year age 11 and 2nd year the year after. Some but not all schools also had a VI Form providing 2 years additional education (sixth form divided into lower VIth and upper VIth)
Years 1-5 are now known as year 7-11 but the name VI Form remains although te years are now year 12 and 13.
Normal progression is that you would sit GCSE (General Certificate of Education) exams in year 11. There are different boards who write and issue the papers but if two schools are using the same board it means all students sitting say GCSE French will take the same exam regardless of which school you go to. Most students take 7 - 10 subjects and the grades are A, A, B, C, D, E, F, G - grades A-C are regarded as a pass by employers and almost everyone else.
Now this is where it gets complicated. No one is ever held down a year so not everyone is ready for GCSE at age 16 and not everyone is good at exams so might take a course work based subject.
So qualifications are
level 1 - equivalent to GCSE grades D-G
Level 2 - equivalent to GCSE grades A*-C
Level 3 - equivalent to A Levels - which I will come back to later.
So you can take a range of level 1 and level 2 courses either GCSE or BTEC (coursework course) or others such as city and guilds.
The 'school leaving age' has just been raised from 16 to 17 and will be going up to 18 but this doesn't mean you have to stay at school.
OK A Levels.
At 16 you have to stay in education or be on an apprenticeship. You can stay on at school and go in to the VI Form, go to the VI form at another school or go to a further education college.
VI Forms are small and mostly offer A Levels. A Levels are 2 year courses that will get you in to uni. They consist of AS - the first year and A2-the second year.
Further Education (FE) colleges are lager and offer a greater range of subjects. They teach A Levels but also Level 1, Level 2, Level 3 and in some cases Level 4 and Level 5 courses (equivalent to years 1 and 2 at uni).
To do a level 3 course you need to have a 'full' level 2 course ie 5 GCSEs A*-C usually including English and Maths. If you leave school without this you can do Level 1 and Level 2 at college. If you want to go to uni in the US this is classed as equivalent to a HS diploma, but any good college wants A Levels which are roughly equivalent to AP classes.
FE Colleges also provide courses in practical things such as hairdressing and motor mechanics.
Anyone from age 14 up can attend college. Many schools work with colleges so students may spend 1 or 2 days a week at college doing a course not available at school.
Adults can also attend FE colleges but may have to pay fees. The rules about fees change every year or two set by government.
So a couple of years ago I did an AS Level for fun.
If you are taking A Levels you normally take 3 or 4 subjects for AS and then continue 2 to A Level, but some people do more.
Other Level 3 courses are equivalent to 3 A Levels and are also taken over 2 years.
I'm sure I have left loads out but that is the basics