This is a great end-of-the-year writing assignment because you don't have to spend hours correcting or grading! In place of an essay for a last marking period writing assignment, students choose a teacher who helped them in some way this year and thank that teacher for his/her assistance in a personal letter. The support the students received may be of an academic nature, but it does not have to be.
I outlined the criteria for this one page letter, but you can amend the criteria requirements to fit your needs. I also wrote a sample, one page thank you letter so the students can use as a model to write their own letters. This is an easy duplexed 2 page lesson, and the teachers receiving the thank you's are always grateful for the personal recognition.