When the administration wants to notify the students of something, instead of just posting a sign on the bulletin board, it has a personalized letter sent to each student's mailbox.
和一般學校不同的是，學校有什麼事要通知大家，不會只貼個公告了事，而是由校長發給每人一封打上個人姓名的信件，由校園郵差送到各自的信箱。