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.
You've done this before, haven't you.