B.,
As far as the teacher's gift, maybe a gift certificate to a salon, spa or massage therapy place if you are going in with several moms. The classroom supply idea is nice, too. When I taught, I really could have used some kind of an album to put the class pictures and also drawings from the kids in. They might like that.
I never gave my kids preschool or Kindergarten graduation presents, never even thought about it. Really isn't necessary. We organized a potluck dinner after preschool graduation, that was enough. If it is going to be a situation where parents are presenting gifts at the graduation, then, of course, you will want to participate. Maybe a book about growing up, (Oh, the places you'll go by Dr. Seuss comes to mind,) a journal where he can draw pictures, a new outfit that he can wear to church later or a picture of his classmates to put in his room.
One thing to consider is the precedence that you set. If you do decide to buy a gift, bear in mind that there will be an expectation for something for every kid for every milestone graduation, and the schools are having them more and more frequently. (We just had High School and college where a gift is a standard, now it's Kindergarten, 5th grade, 8th grade, etc.)
Good luck,
S.