That is totally wrong. I have both of my kids through these private schools, and their curriculum sucks compare to the China one. "The 6 year old sucks in math" is really true because all international schools sucks in math compare to China's schools. However, the nice thing is there are tutoring companies like XueErSi that offers high end classes. (Pretty much banned a few years, but it is now back, also government is starting to support these classes this year) My kids learn math, computer science, chemistry, physics from XueErSi to compensate their private school education which is only good for English.
I always question UN's HDI, because a large part of it is "Mean years of schooling and expected years of schooling", and because of China's 9 years compulsory education, China lost a lot of points. However, I don't understand why UN don't understand that what is important is how much students learned, not how many years they spend in school. Chinese in China's schools learned more in 9 years than most western countries' students learned in 12 or 13 years. Just compare China's 9th grade student's math skills to adults in US or UK or Canada, and you will see.