Not quite. Here's the complete educational timeline:

Infants school: 4 - 6 (5 - 6 in some cases)
Junior school: 7 - 10
Senior school: 11 - 15
College or 6th Form: 16 - 17
University: 18+

Everyone has to take at least some GCSEs (the modern version of O Levels and CSEs), over a period of two years at the end of senior school. At the end of that time (when most students have just turned 16) you can leave school altogether if you want.

If you want to continue your education you can leave school and go to college (NOT the american meaning of college) for two years to take A Levels or a vocational equivalent. Alternatively you can stay in school for two more years to take your A Levels, which is known as 6th Form for historic reasons. Over the last year or so most A Levels have been replaced with AS Levels, which are courses with about half the content of A's (so you do twice as many of them). That is in line with current thinking that 16 is too young an age to specialise.

After that, you can go to university if you think you can handle the beer.

Rob