Sashidashinin Wa, Daredesuka? (差出人は、誰ですか？)

Storyline

The stage is Arakawa Nishi High School, where the main character Mizuki Kuwatsuru (Sara Kozawa) attends. On the first day of the second semester, her homeroom teacher, Tachibana, starts a "letter game". Her rule is to write and mail letters to her classmates. You don't have to write the sender, but it's a rule to write your true intentions.