I send these postcards to students systematically throughout the year as a way to encourage them. I like the idea of notes, but postcards are cheaper, and have the added bonus of being seen by parents. Parents and students enjoy them, and there's just enough space to add a personal message on the back.
The back side has lines dividing the message from the address and says "I am so thankful for you!" at the top of the message side.
They print on 8.5 x 11 cardstock; then flip and print out on the other side [make sure to test your printer configuration first!]. Then cut them with a paper cutter. Voila!