Why single Shanghai out? What about the average for all of the PRC?
OCED did the PISA, and as you know OCED is the organisation of a bunch of highly developed nations, their education system is fully developed and is unlikely to improve drastically in the future. Whilst China, although economically, second largest in the world, is still a developing nation, and much of the country education system is not fully developed, so to compared China as a whole to say UK as a whole would not be fair, a lot of the area in China such as Tibet, Gansu (the western parts of China), children can not afford school etc.
Shanghai's education model is like to the future for the China's education model, and Shanghai's economy is on par with that of OCED countries, so it is fair to compare only Shanghai to the rest of OECD instead of PRC as a whole.
PS. my home country Australia = fail