After a class size of 50 was considered “standard” in the compulsory education standard act enacted in 1958, the size was reduced to 45 in 1964 and to 40 in 1980. Since fiscal 2001, however, class composition has been left up to prefectural boards of education. Prefectures except Tokyo currently have 40 or less students per class, but this is mostly limited to early elementary classes.
http://mdn.mainichi.jp/mdnnews/news/20100115p2a00m0na005000c.html
