I did a small gift for my daughter's MDO teachers last year. I usually do an african violet or other small plant with Christmas colored foil around the pot and then make a small flag to stick in it with a bible verse on one side (or just Merry Christmas) and then I put "Thank you for helping Abbey grow". This is a good gift for either Christmas or the end of the year, but obviously you wouldn't do it for both. I think the end of the year gifts for the teachers tend to be a little more elaborate (at least from what I experienced last year), so I like the plant idea for Christmas.
They will probably get lots of candy and cookies and stuff, so if you aren't keen on the plant idea, I would do a gift card to Barnes and Noble, Starbucks or a restaurant. I usually keep it around $10, but the amount would probably depend on what you're comfortable with and your family budget for the holidays. Good luck! I was new to this last year, so I know where you're coming from. I still don't think I have it figured out.
V.