Message Received?

Starting on January 14th, the student council will use an empty music room to hold a Valentine Mailbox center. People can send letters, gifts, or whatever else they choose to be delivered to their special someone. They can attach their name, or remain anonymous. They can have it sent anytime during the Valentines celebrations. They also have other special games after school. Jeongin's friends made fun of the idea, so when he starts getting anonymous letters a week before Valentine's Day, he doesn't say anything. The letters are always neatly typed out on pastel blue paper, and the sender says they won't come forward unless he confronts them in the next seven days, even if he guesses correctly later. He has a few suspects:His friend, Jisung, the biggest theatre nerd the city has ever seen.Changbin, captain of the football team and the school's resident closet case.The senior he's always been close to, Chan.And Minho, who's been tutoring said senior.So who will it be?