Before and during World War II, Japanese education was based on the German system. During the occupation in such period, the educational system was changed due to such foreign influence to abide more with the United States educational way. A main change was that junior high became mandatory.
Therefore the correct answer is:
World War II
Wikipedia (2007). Occupied Japan (on line). Available from: http://en.wikipedia.org/wiki/Occupied_Japan (Accessed 13th June 2007).