IMO it all depends on how close are you with the teacher, i mean, are there lots of ppl in the group where your son is, have you contacted her much before or you just wanna give her a little present.
Teachers are the women like all of us! Back in the days when i was entering my college and passing the exams i had an english teacher, we had a group of 4. So me and my Mom got her a Trussardi set of perfume and body lotion. And i was lucky, it worked out fantastic cause she loved the scent and even called us back to sat thanks! You can try getting her sonething for bath, or make-up, something not very expensive, but cute. Or maybe something for household, some nice home decorations etc? If you dont come up with any idea, you can just bring her flowers and cake..