The highest performing OECD country overall, South Korea, scored a mean of 539. The lowest performing, Mexico, scored a 425 mean. That difference means that at age 15, South Koreans have packed in the equivalent of two years more worth of schooling than their Mexican contemporaries.
The difference is even greater if the analysis is expanded to partner countries and economies. Shanghai-China is at the top with a 556 mean and Kyrgyzstan is at the bottom with a 314 mean. That is a 242-point difference, or six years of schooling, according to PISA.
The OECD average is 494. The US mean score was close to the OECD mean.