Vistaprint is offering 100 free back to school postcards for teachers! This is a great way to welcome new students...depending on your class size, these could last you a few years! You do pay for shipping, but it's only about $5-7. Go to this link and then go down to standard postcards, and hit get started. Type in "School" or "Teacher" or something you want on your postcard into the search box, and you'll be presented with all kinds of options!
Make sure they are color on one side and black and white on the other, those are the free ones. Then when you check out, make sure you don't click any extra offers. And then that's it! 100 post cards, free for shipping!