Other answers have alluded to the key issue her, namely the difference in educational culture that exists between th UK and many pacific rim countries. Rightly or wrongly, the best of UK education at present is aimed at developing collaboration, thinking and problem-solving skills. The old model of western education which spread across the globe is based on rote-learning, memorisation and repetition (in writing) of approved 'facts. This is the model used in China, for example.
Allied to this is the simple fact that the societies of China, Japan, Taiwan and Korea are exceptionally conformist. There is simply no notion of being held back in class because of another pupil's bad behaviour, and everyone from parents downward conforms to rules and customs. I have seen primary schools of 2000 primary schoolchildren, with classes of up to 60, in Taiwan. There was no notion of any child being given special help, as any child not able to chant along with the rest would be ignored by the teacher and his peers until he stopped attending school.
And that too is a factor that skews the statistics - ie as schooling is paid for by parents, not freely provided by the state, there is no recording of or concern for the children who fail to attend - they do not exist statistically, and so the children who attend school are all 'bright'.