Now, I'm not a teacher, so I'm just going on generalized thoughts regarding a thank you for a Christmas gift. If the class as a whole pitches in for something, maybe a verbal thank you to the group is sufficient. If individual students give you gifts, I'd be more prone to send a card to them. Altough, nothing too formal, observing the boundaries, just a respectful "thank you" note.

If I may ask, as a teacher, how many coffee mugs do you own that say #1 Teahcer in goofy font? (it's even better if the mug came with Hershy Kisses or some other candy inside!)