PT-ratio in schools has been decreased: from 38.2 (1948) to 16.5 (2012) in primary schools. The changes of PT-ratio depended on that of number of students. PT-ratio was decomposed into school and class size components as follows:
PT-ratio = number of students/number of teachers
= students/class * class/school * school/teachers
= class size * school size (n of class)/school size (n of teachers).
The relationships among PT-ratio, class size and school size were examined in primary, junior high and high schools. At first, improvement of PT-ratio was closely related to the improvement of class size in primary and junior high schools. Secondly, it was closely related to the improvement of school size (n of class) in high schools. Thirdly, a unique phenomenon was found in recent several years: school size (n of class) increased in spite of the decrease of number of students. It is considered that many local governments began consolidation of small schools on the aware that the reduction of number of students would be a long trend in their community.