I didn't come up with this myself. It actually comes from a sixth grade teacher in Texas, but it works well anywhere!

What student isn’t at least a little apprehensive about his new teacher?  On the first day of school, read aloud Mike Thaler’s short book: The Teacher from the Black Lagoon.  After reading the book, have students discuss the qualities of a good teacher.  

Ask, “What little piece of advice would you give me about how to make this a positive year for everyone”?  As a follow-up to the discussion, have each child write a letter to his favorite teacher telling why he or she was so appreciated.  (For the younger students, it might be necessary to rely on illustrations as the focal point of the letter, rather than full sentences. You might even write a sentence on the board, for students to copy, such as: "Thank you for being my teacher, you really made a difference!")

Since the letters will likely contain mistakes, use this activity as an opportunity to reintroduce the writing process steps of editing, revising, etc. When the letters are complete, send them (use your school system’s interoffice mail system if students have changed schools).  

How encouraging for any teacher to receive a letter of praise at the beginning of a new year!