I used to like you

That is the first and the last letter that Jungkook receives in his locker.

7 years have passed since his Highschool graduation and Jungkook is now a president in a well-known company. Then one night he receives an email saying that he's batch will have a high school reunion will Jungkook be able to find out who placed that letter in his locker at that time or will fail?