About one month ago, OECD published the result of the Survey of Adult Skill (PIAAC) and Japan demonstrated a very good performance there.
According to the Ministry of Education of Japan, Japan topped the other countries by a relatively big margin in the average score.
I'm not sure to what extent this kind of international comparison (competition) is meaningful, but I'm sure that no official or unofficial interpretation, explanation or analyses of this result will take into consideration the effect of supplementary education in Japan.
By the way, there is a group of education academics who praise the Finnish education in Japan as well, and they have insisted that the Japanese education should follow the Finish model. Their argument very much depends on the result of another international educational assessment (PISA), in which Finland was top. I wonder how they justify their argument with this PIAAC result. This may become another taboo topic.