I love the thank you cards that Rachel M. posted below. Here's a gift idea to send along with the cards to the kids' teachers. It is a pretty and thoughtful gift and very inexpensive-perfect if you have many teachers and teachers' assistants on your gift giving list.
I bought small pots of thyme at my local hardware/garden center (think Home Depot or Lowes) and wrapped the pots in tissue paper and cellophane. I tied raffia, in a bow, around the center. (For a more organic look you can wrap the pots in burlap.)For each pot, I took a scrapbooking tag and wrote on it: Thank you for all the "thyme" you spent teaching _________ this year. I glued the tag to a colored pop stick and placed it in the pot.