Quote from LawX
Confessing in letters isn't a strictly Japanese thing. People will do that no mater where they are from.
the mangas that i read make it seem like in high school that is the main way to confess to someone. maybe it's just where i went to school, but when i was in high school i don't remember anyone confessing in a letter. although people would write love letters once they were a couple (i do remember people writing letters to confess in elementary and middle school, but not many)