The group gift is a great idea, but as someone who works in an elementary school, I can say that teachers love gifts made by their students. I have kept all of my cards and pictures in a special photo box and I love to look through them, especially if I've had a rough day.
The kids are so genuine in their thoughts and it makes my day to have them hand me something they have made and are so proud of.
Have your little guy make a card, even if all he can write is his name. Suggest that he draw a picture of his favorite thing to do at school and if you can include a recent picture of him somewhere in the card or on the back, that would make it even better.
Then you can toss a small gift card to Sonic or Starbucks inside it if you really feel like you need to (even $3-5 is enough). Put it all in a cute gift bag!
Trust me, good teachers appreciate the thought, not the amount spent. Have a wonderful holiday!