At the end of the year one of the things I do is go to Target and buy 100s of Thank You cards. I think the pack I bought this year was $2.99 for 50. I ask students to write a thank you to a person at the school. It is usually a teacher but can be anyone. Our "student advisor" (AKA the huge guy that yanks kids out of class) gets a lot of cards as do the people that run the afterschool program. My only rule is that it can't be to me. Some kids do only one. I think this year one student wrote five. Then I drop them in mailboxes. It's a nice thing, especially for the 6th grade teachers who assume they have been forgotten.
I showed my students the Neil Tyson "Most Astounding Fact" video as the introduction and mentioned that this is where my introduction came from. Since I asked them to write Thank Yous yesterday I also included a thank you. Then I just gave them the letter. Like Sam, ideally I'd like students to keep it in their yearbook and randomly stumble on it every few years.
I tried to keep it to one page because I'm not entirely sure 8th graders will read even that much.
I over-edited and the transitions are ugly. I also took out the more personal stuff in favor of broader stuff. I think that was a mistake. I should have definitely left in, "Future classes will look up at the ceiling, see the smoke stains, and know that we were here."