こーかんにっき https://t.co/DkgLJZvo2r

Exchange Diary https://t.co/DkgLJZvo2r

Via Wikipedia: An Exchange diary (交換日記 Koukan nikki) is a notebook shared between friends, who take it in turns to write in their thoughts or other comments. Exchange diaries were especially popular in Japan in the 1990s in elementary and junior high schools, particularly among girls.

https://en.wikipedia.org/wiki/Exchange_diary