(courtesy of the Lake Oswego Oregon Stake, and shared by Bobbie Poppleton)
Have all the children get off their chairs and push them back. Have the entire class join you sitting on the floor.
This is a great way to grab their attention to tell a story. All eyes will be on you. Tell the story in a small voice, with lots of facial expressions. If it is successful, you could finish out the lesson on the floor (assuming you can get someone to pick you up off the floor at the end).