This is book number 23 in the Magic Tree House series.
In the third book of the American history cluster, Jack and Annie are whisked to a midwestern prairie in the 1870s. They visit a one-room schoolhouse and learn about the hard life of the pioneers. When they return to the magic tree house, Jack and Annie spot a twister on the horizon. Is there time to warn the teacher and the children back at the schoolhouse? Or should Jack and Annie just save themselves?