In addition to the summary from the article posted by Tad Watanabe (note that teaching methods are *not* one of the reasons the authors cite), I found the following little gem.
Cumberbatch and Butcher say the system is not perfect. Only about a quarter of Barbadian students get the chance to attend two years of pre-university work at a school such as Harrison; one in 10 get to a university.
This raises an additional question. What is the population taking the SAT in Barbados? Perhaps if we only looked at the scores of our top 25% (or less) students, we would have equally remarkable scores.