He’s trying to teach you something about life after college. He’s trying to prepare you for the less casual atmosphere of many workplaces.
I’ve worked in my school for over 25 years, and am on a first name basis with my Assistant Principal. (The principal is a member of a religious order, and is always addressed with his title.) But any memos to or from the AP are addressed to Mr. AP from Mrs. bjkmom, and have the standard To; From: Date heading.