Re: Explan me the whole GCSE system. O'levels, A'levels, etc.
To answer a few of your questions, the GCSE system is applicable only at the O-Levels stage and is available in the U.K. along with some other commonwealth countries through the British Council. A lot of other commonwealth countries still get examined on the G.C.E. system which is applicable at both the O- and A-Levels.
The exams are administered all over the world and students are tested on a common curriculum albiet questions that are set are different. Last I heard, each continent gets the same exams... and this was only done after several discrepencies emerged in the 90s' when questions started being faxed over from one place to another right after an exam was conducted in a region with a negative time-difference (Pakis ofcourse in the forefront as always :) ).
As far as getting As is concerned, the scale for awading specific letter grades is flexible. The year I wrote my A-Levels, an A on the Business Studies exam was awarded at a 90% !! while on the Physics exam, it was at the 75%. Again, you can request a detailed marking scheme from the British Council once the results are announced.
As far as performance in the subcontinent is concerned, students almost always score better there as compared to anywhere in the west. Again, this is because our educational system has different techniques with particular emphasis of learning by rote which in my opinion works well with the O-Levels and A-Levels exams. Furthermore, its just my opinion but I think there are a lot of schools in Pakistan that train students through practical tactics and directives for effectively answering questions based on patterns that teachers have identified over the years. This also works well with the O-Levels and A-Levels.
Most institutions in Pakistan accept the A-Levels at least at the Inter level... some universities set requirements for entrants to write other exams such as Pak Studies, Islamic Studies etc. in order to gain admission, but from what I've heard lately that's changing in Professional Schools. Some institutes even accept O-Level students directly into their programs - e.g. back in the days, you could directly get into PAF's Aeronautical Engineering programs after O-Levels as opposed to having to do Pre-Engineering/F.Sc.
Finally, having experienced the British education system (I did both the O- and A-Levels), the Pakistani system (tried the F.Sc. route for a year) and the North American system, I honestly believe that the O-Levels and A-Levels leave all other systems behind in many aspects. Most students who score a B or an A in their A-Level subjects typically get exempted from freshman year courses in North America... they also find the transition from high-school to University much easier as well.