According to Japan Press Weekly
An annual survey conducted by the All Japan Teachers and Staff Union (Zenkyo) indicates that more than half of all public school teachers work at least 80 hours of overtime every month, the government-set danger line for possible karoshi (death from overtime). Figures in the survey included no classroom hours worked on Saturdays.
In August this year, an elementary school teacher in the city was arrested under allegations of sending the city education commission “threatening letters” demanding an end to Saturday classes.
In the “letter”, the teacher wrote that Saturday classes have led to constant fatigue and may cause his death.
University of Tokyo Graduate School of Education Professor Katsuno Masaaki pointed out that imposition of longer school days will produce total exhaustion upon teachers who are already working excessively long hours. “Providing a better teaching environment for teachers, including smaller-sized classes, is essential to improving student achievement,” he stressed.