回复：留学生入关后必须10天之内到学校报到吗？

什么时候报到，取决于学校要求（when F1 starts）。

但你侄女可以在F1 开始前30天内入境。

http://travel.state.gov/visa/temp/types/types_1268.html

Students are advised of the Department of Homeland Security regulation which requires that all initial or beginning students enter the U.S. 30 days or less in advance of the course of study start/report date as shown on the Form I-20. Please consider this date carefully when making travel plans to the U.S.