Hundreds of high school students laugh and sing as they pitch in to place 100,000 sandbags around Fargo to help protect homes against Red River flooding. Defense Day, as the classroom hiatus has come to be known, began in 2009 when the city fought the first of three straight major floods. Students placed 700,000 sandbags in less than two days during the last flood in 2011. They're at it again this year.