I think the main difference is that in USA everyone stays on til they are 18. We don't. Kids here can leave at 16, having taken their GCSEs- between 8 and 12 subjects usually.
Then they can either get a job, go into some sort of training, go to the 6th form (usually back to school but they don't have to wear a uniform etc) or go to a FE College. (Further Education). An FE College is the same as 6th form except it is not in a school- so there will be more adults learning, a variety of vocational courses- BTECs rather than just A Levels, and things like hairdressing, catering, building, plumbing etc. Students can learn a trade rather than studying academic subjects.
When I was 16 I stayed on at school cos I was happy there, in the 6th form. All the rebels in my class who hated school went to FE College where they could smoke and call their teachers by their first names. College is much more "free" usually.
After the 6th form (usually at 18) students can either leave and get a job or go onto university (HE- Higher Education). University is 3 or 4 year courses.
There is talk about the government raising the school leaving age to 18 though.