your best friend has just been suspended from school for truancy write a letter to him or her expressing your displeasure and advising him or her to turn a new life​

Since this is an informal letter to a friend, addressing the issue of his suspension from school for truancy, only the writer's address, for a possible return mail, is important if you are using slow mail.

However, an email will do the magic these days.  The (letter) email will automatically indicate the date of writing and also show your email address.

Begin the letter with a suitable salutation like this: "Dear Bose."Introduce the reason for writing.  For example, "I learned from Mike, your assignment partner, that you have earned a one-week suspension following your usual truancy."Give a general preamble about how you became aware of his suspension due to truancy.Write about how you have tried to talk your friend out of the poor habit of truancy.  Reassure him that his interest in having a bright future can be made possible if he starts doing the right things at the right time.Tell him that you wish to discuss more when he returns to class.  In the meantime, encourage him to be of good behavior and not to engage in any activity that will disappoint his parents or put him to ridicule.Close your writing by reassuring him that he remains a friend.Indicate your name.

This is an informal letter to a friend, and not a business letter, which will naturally require more formalities.

Learn more about truancy at