Strict academic research on the historical aspect of school durations around the world is rare. However, there many possible socio-economic and cultural factors at play from which to piece together probable explainations for longer school days and school years in Japan versus America.

One possible factor is Japan's emphasis on having one of the world's best-educated populations.  The Ministry of Education closely supervises curriculum, textbooks, and classes and maintains a uniform level of education throughout the country, with an aim of 100% enrollment in compulsory grades and zero illiteracy. While not compulsory, high school (koukou 高校) enrollment is over 96% nationwide and nearly 100% in the cities. The high school drop out rate is only 2%, though it has been increasing. About 46% of all high school graduates go on to university or junior college.  As a result, a high standard of education is possible, and the Japanese put a high value on it culturally. 

Another possible reason is cultural influence of outher countries educational systems.  While it is true that the Japanese educational system was reformed after World War II, to be more in line with the American school system: the old 6-5-3-3 system was changed to a 6-3-3-4 system (6 years of elementary school, 3 years of junior high school, 3 years of senior high school and 4 years of University) with reference to the American system. However, the modern Japanese educational system's roots actually extend back to 1872, modeled after the French school system which begins in April. The fiscal year in Japan also begins in April and ends in March of the following year, which is more convenient in many aspects.  April is the height of spring when cherry blossoms (the most loved flower of the Japanese!) bloom and the most suitable time for a new start in Japan. This difference in the school-year system causes some inconvenience to students who wish to study abroad in the U.S. A half-year is wasted waiting to get in and often another year is wasted when coming back to the Japanese university system and having to repeat a year.  Further explaination may come from neigboring Asian influences, such as China, most of which have more intensive school days and years than the American system.  Except for the lower grades of elementary school, the average Japanese school day on weekdays is 6 hours, which makes it one of the longest school days in the world. Similar to other Asian countries, even after school lets out the children have drills and other homework to keep them busy. Vacations are 6 weeks in the summer and about 2 weeks each for winter and spring breaks. There is often homework over these vacations, unlike European school and work systems, which put a high value on quality vacation time.

Anoher factor in the increased length of the school day in Japan may be making the best use of limited reseources.  As a result of the rapid population growth after World War II, the numbers of students in a typical elementary or junior high school class once exceeded 50 students, but now it is kept under 40.  And every class has its own fixed classroom where its students take all the courses, except for practical training and laboratory work. During elementary education, in most cases, one teacher teaches all the subjects in each class. Longer school days may be a way of making sure each student has access to everything they need, as time is split between limited resources such as teacher attention and classroom space.

Lastly, a big difference between the Japanese school system and the American School system is that Americans respect individuality while the Japanese control the individual by observing group rules. At public elementary and junior high school, school lunch (kyuushoku) is provided on a standardized menu, and it is eaten in the classroom. Nearly all junior high schools require their students to wear a school uniform (seifuku). This helps to explain the Japanese characteristic of group behavior, which makes consensus on longer learning durations and working hours more pallatable to the Japanese populace than it is in the diverse and staunchly independant American school and work systems.
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