YOU SAID:

I have to go to school for the next fifteen days.

INTO JAPANESE

私は次の15日間学校に行かなければなりません。

BACK INTO ENGLISH

I have to go to school for the next 15 days.

INTO JAPANESE

私は次の15日間学校に行かなければなりません。

BACK INTO ENGLISH

I have to go to school for the next 15 days.