In the UK, after you have left school at 16, you have a choice to either get a job, or go on to do further education at either a college or a 6th form school.
There you do another 2 years of education, to gain more qualifications.
At the end of the first year, you take exams called A levels but at A/S level, this equates to half an A level.
To complete the full A level, you have to do another year of education, and then take your full A levels at age 18.
When you have completed your A levels, most people who have taken them go on to go to University.
My eldest Son is taking his last A/S levels today. They have been taking these exams for several weeks now. After this, he will get a break of a few weeks before he goes back into year 13, to work towards taking his full A levels.
The work at this level starts to get even harder than it already is.
I found a few good sites to help out those preparing for the AS/A Examinations.
https://www.aqa.org.uk/qual/gceasa/comp.html
Also, Amazon.com has a ton of books that will help you prepare for these tests.