Empathy and sincerely asking for forgiveness are important lessons that middle or high school teachers don't always have time to explicitly teach. This assignment is great for reinforcing these principles in addition to having the student practice their writing skills.
To successfully assign this task the teacher needs to write the student's name and the date at the top of the letter then a brief description of what the student did wrong. (Examples: disrespected a substitute teacher, stole supplies from a teacher or peer, lied to an administrator...etc.) Finally, the teacher signs their own name and title at the end of the letter.