Okay, so lemme see if I got this straight. At 16, you could choose to leave school, and if you did so, then you could take O-levels or CSEs (or, after 1988, GCSEs) or neither. If you decided to stay in school, then you did so for two more years, and then you took A-level exams, and they functioned as college entrance exams. Is that right? The only place where I suspect I'm wrong is that the people that stayed in school from 16 to 18 took the O-levels as well.

And all of these exams are separate for each subject, right?

Just to compare, in the US, we have grades 1 though 12, usually divided up where 1 through 5 are ``elementary school'', 6 though 8 are ``middle school'' (formerly ``junior high school'', and 9 through 12 are ``high school''. Each year, you get a single grade, for lower grades, or a grade for each course, usually 6 or 7 courses per year, in higher grades. In order to graduate to the next grade, you must score well enough on the current year's grade. If you don't, you get ``held back'' and have to repeat the grade. Once you progress through all 12 years, you get a high school diploma. This is when you're 17 or 18. There is no final exam to get this, other than the final exams you'd take pretty much every year for every class. You can drop out of school when you're 16 here, as well, but you get nothing. You can later get a GED, which is a high school equivalency diploma, by taking a test or series of tests (I'm not really sure. As far as I know, I've never known anyone with a GED.). Oh, and there are CATs (California Achievement Tests) taken every few years or so throughout grades 1 to 12. These are standardized tests that, I think, have no bearing on your grades, but end up being used by the government to determine how schools are funded.

There are two standardized college entrance exams, the SAT and the ACT. I've got no information on the ACT; supposedly, it's more common on the west coast. The SAT consists of, IIRC, 5 exams -- 2 ``verbal'', judging language comprehension, etc., 2 ``math'', judging math skills (I think through basic algebra), and one other, either math or verbal, that's usually a test for the next year's exam, and doesn't count towards your score, but you don't know which one it is. These are all taken during one morning for 3 hours or so. The scores on these tests (you get one math score and one verbal score -- the individual tests are not counted individually -- each up to 800) are large deciding factors on college applications. You can take the tests as many times as you want, I think (there might be some limit, but you can definitely take them two or three times), but they cost some amount of money to take ($25 or so?). There are also AP (advanced placement) exams for individual subjects that don't really have any meaning on their own, but some colleges will let you place out of early courses if you score well on them. These exams are not given at the same time at the SAT (usually), but usually if you take multiple ones, they will all be on the same day.