That's a very bad assumption. Starting dates vary wildly from school to school, much less from country to country. For example, your sister starts this month in a few days, whereas mine starts at the end of next month. Through a little bit of research, you can easily figure out exactly when she starts:
We know she's at Keio SFC, and they have their website
in English!
July 27 - September 21 Friday - Friday Summer Break
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2007 Fall Semester
September 21 Friday Matriculation Ceremony: Mita Campus
September 20, 22 & 25 Thursday, Saturday & Tuesday Orientation
September 26 Wednesday Classes Start
Looking at the calendar, the school has make-up dates for classes and final exams. More proof that Japanese schools are flexible and/or easy?