Kazzyhoward
I did O Level and CSE Chemistry, I got a grade 1 CSE and scraped a C in O Level.
daft comment.
To take an A level you need to have acheived a certain mark, eg a 6 at GCSE.*
No you don't. Lots of subject are not available at GCSE or not available at all schools, Computer science is one.
For languages it's quite common for a student to add a different language so they may have GCSE French and German but pick up Spanish but be in a class with students with GCSE.
Psychology, economics, sociology, law - not many schools teach them at GCSE.
Lots of students do double science and go on to take A Levels in science subjects.
At uni there are a lot of courses where you don't need a specific set of A Levels and you will share the course with students who have taken BTEC, IB, Pre-U, access courses and mature students who may not have taken a level 3 course at all.
Some universities put groups of students together for individual modules so an anatomy class may have students doctors, nurses, clinical physiologists and anatomists.