My suggestion would be a beaded necklace for female teachers, or bookmarks with some special message on them and then laminated for either male or female teachers.

Hope this helps

Last year my daughter made a plaster hand print for her teacher. I used a toothpick to write in the year and who it was from. I then let my daughter paint it. We wrapped it up in bubble wrap and then cellophane. Her teacher loved it!
Other ideas are a thank you card with a photo in it. The card could be in the shape of an Apple or the ABC’s. You could also do a small jar or cup with some lollies in it.