I'm not sure Shenzhen, Beijing, or Shanghai is the top performing city in China's own national college entrance exam (Gaokao). I'm not sure Jiangsu and Zhejiang are the best performing provinces in China's Gaokao either. There might be better performing cities and provinces in China than those. On the other hand, I think one of the most important reasons that B-S-J-Z were chosen to participate in the 2018 PISA was because these regions matched the conditions of the core OECD nations best in terms of income and development, more so than other poorer regions of China.Why don't they test the nerds at Shenzhen? These people might even beat Beijing and Shanghai there.