My girl friend bought her daughters teacher a 31 bag for christmas. I guess these are At home party type deals so a pretty pricey bag. but she got a great deal wtih something she was already going to buy so it wasn't as much as you would think.
i've heard of people getting Coach bags for the lead teachers in wealthy areas. i'm sure that will make people fall over, but some people just like to give gifts and in my opinion Who better to show your appreaciation to.
i always think it' helps if you know the teachers likes, or hobbies. when i taught preschool, again wealthy neighborhood, I had a student that didn't celebrate the holidays but at the end of the year Mom wanted to show her appreciation, she knew i liked stamping and making my own cards, She bought a Longaberger basket and Filled it with rubber stamps and supplies, IT WAS AWESOME! 14 yrs and i still remember that little guy and this lovely lovely family, they were very involved in his education and sooo suportive, i would have remembered them regardless, but it bowled me over to know she took the time to find somethign that i would love.
obviously with all the teachers your son has you might not want to go that big but i hope you get my point that the gift was personal.
Providing Lunch for the teachers or heading a group of parents to help with that might be a lovely idea, you could have it catered or other mom's join you for a potluck. tableclothes and flowers in the teachers lounge would be so fun.
one last idea. when my own son was in preschool, I had him water color on some nice heavy paper I really let him soak up the colors so they were very vibrant, I also had him draw each of this three preschool teachers separately and dictate to me what he enjoyed most about each of them, One loved science, one tended to be more musical and one helped him with his art projects. I mounted his drawing on to the water color paper and bought frames for each. If they chose not to keep it as a momento they could always use the frame for their own picture. Depending on your sons ability you could use a photo of him, or you could even buy a frame that lets you record a brief saying and have him say " THANK you Ms So and So for helping me to hold my pencil or what ever."
Anyhow, I think you are so nice to be thinking about this early. I hope you find the perfect thing!