Try not to use duct tape or stiff packing tape. They hurt when they come off and bring some skin with them. Get a roll or two of the white cloth athletic tape. Most high schools use this stuff in the athletic trainer's office and/or PE departments. School nurses usually have some on hand also.
Just tape the kids thumbs to the side of the hand (or just a little piece around thumb and index finger for the little 'uns...) so the thumb is now inoperable. Heave the kids try to perform simple physical tasks like sharpen pencils then write their feelings, clean the desk tops, use a computer mouse or keyboard, even picking objects up is a challenge. For the older kids, have them keep the tape on for a day then listen to the stories of trying to drive without a thumb, brush teeth, comb or curl hair, putting on make-up, just going to the lavatory is a challenge!