Answer:
Hi! So, the informal tone you would use for your bestfriend, your younger cousin, and a classmate, because you know these people personally and their your friends or family members.
The formal tone you would use for everyone else: the school principal, a religious figure in the community, and the mayor of your city, because you do not know these people and you want to be as respectful as possible. Do not talk to these people like you are talking to your friends.
Explanation: I hope this helps, have a great day! :)