Keychains or pins that recognize volunteers with an inspirational message, such as "Volunteers Make a Difference," "Volunteers are the Key," or "Volunteers Measure Up." Although you could find these items at a local store that customize pins, the service can be costly. It might be easier to go a online where they can be found for a few dollars.
Take a class photo with a camera. You can do a serious photo with the students or a silly photo. Have the photo enlarged to an 8-by-10 image and have the back signed by the students (and teacher). If there is space, each can express their gratitude. Add a nice frame.
Thank you cards are a simple gift that can also be the most meaningful. There are many ways to make thank you cards, but basically, you can hand each student a piece of paper and have them fold it in half. Inside the cover, have the students write a message about how the volunteer help them. Have the students decorate the cards for that final, personal touch.