The first four years at secondary (high school) school is spent studying for “O” (ordinary) levels. They are now called GCSEs (general certificate of secondary education). In the first two years, you take basic subjects, such as maths, english, science, history. Compulsary subjects for first and Second year vary from school to school. At the end of second year, one chooses at least two extra subjects to take, such as French, business etc. This is much like the way Hogwarts works: you have to keep your old subjects and you take on new ones. You can take as many as you like, but most people take three extra, making 8 subjects (5 core subjects + 3 extra). At the end of fifth year they take an exam or series of exams in each subject. Each subject is one “O” level. If you pass (grade D or above) then you have one “O” level. So if you take twelve subjects (I know someone who did this), and pass, you have 12 “O” levels. This is much better than someone who only “got” (passed) 5 “O” levels. Hogwarts seems to follow this. Really and truly, taking more than twelve would be lunacy. There’s a lot of work that needs to done for each subject and there just aren’t enough hours in the day, as Hermione found out!
The next level of exams are “A” levels. These are studied for in sixth and seventh year. This is where is gets even more specialised. Again you can take as many subjects as you like, but this time you only take your chosen subjects. For example, I did Drama Studies, English Lit and Theatre Skills, and those were the only subjects I did. This is because the courses are very in depth. At the end of seventh year, you take the “A” Level exams, and again each one you pass is one “A” Level. You get points acording to the grade you get, but usually universities specify the grades they want you to get for their courses. For example, if you want to study pure maths at university, they will specify a grade B in maths and two Cs in your other subjects. I don’t know if this is how it works at Hogwarts, as the books haven’t gone into detail about this yet. All we know is that Percy had to get top grade N.E.W.T.s to get into the ministry. I assume this means they have a mimium grade requirement for jobs.