I can explain it.
The norm is
Primary school which goes from reception (,optional) ~4 years old to year 1 through to year 6 ~11 years old.
Then there is secondary school which does school years 7-11. In year 11(age 16) kids will do GCSE (general certificate of secondary education) exams in around 11 subjects. Some kids leave School at this point.
Others will go to 6th form (school years 12 &13) where they will normally do 3 or 4 A levels (Advanced levels).
6 form colleges can be separate schools but often a secondary school has its own 6th form so the school caters from school years 7 to 13. (11-18 in age)
At 18 those staying in education go to university.
That is the basic set up.
Private schools and possibly some areas (the structure often depends on the area) can mess around and have primary schools, prep schools then secondary school. But the basic is primary to age 11, secondary to age 16 sixth form to age 18
Grammar schools are a secondary school with entrance exams those who fail go to a comprehensive secondary school
"6th form" follows the old numbering system that started with secondary schools covering years 1-5.Secondry schools have now changed to years 7-11 but sixth form has stuck.
GCSEs used to be called O levels (ordinary level).
Highlghting mine, clarfication below
1
Not all areas have Grammar schools, they have just Comprehensives.
Then of course we have the Academy schools that are taking over. Some are like Grammar schools in that they are selective, others aren't. They are semi privatised in that they are not controlled by local education departments and are run by a Trust that can have several schools in it.
Then there are the specialist schools. They are like the Academies but they specialise in one subject area like sport or engineering.
There are also faith schools like Church of England, Catholic and Muslim schools mixed in
There are also the new Tory 'Free Schools' that have complete self control and aren't subject to the same regulation levels as other schools and don't even have to employ qualified teachers.
Also, sometimes Primary education is split between separate infant and junior schools.
An alternative to sixth form is Further Education college
2.
1It should be noted that it is the law now that everyone must now stay in education up to the age of 18 (be that school, FE, or an apprenticeship).
2most FE colleges also offer GCSE and functional skills courses for those that have not achieved these at school, for whatever reason.
See? Simple, innit?
ETA- to use my own education as an illustration (for context, I was born in 1975 [so something of a spring chicken around here], and in Kent [which still has selective education]):
St. Alphege Infant's School,
The Endowed CoE Junior School,
Simon Langton Boy's Grammar*, up to half way through sixth form (when I was 'asked' to leave,),
Canterbury College (two years of A-levels, kind of ****** them up - discovered girls and other intoxicants),
A year of crappy jobs and dole,
Canterbury College (again) 2 year GNVQ - only funded for one year, so done in one year. That was fun.
Thames Valley University.
*As I had friends at various local secondary moderns, (there are a lot of schools in Canterbury) I was able to measure the facilities and breadth of sylabus of my historic, storied grammar school against their dirty, common little 'comprehensives'.
Suffice to say, I regretted my choice.