Nearly all of the 31 killed when a Bangladeshi fighter jet crashed were children. Dozens more were being treated in burn centers as the nation declared a day of mourning.

The third- and fourth-grade schoolchildren were settling into a typical lunch break in their classrooms on Monday when the plane crashed into their building.

A fighter jet had suddenly flipped in midair moments before it slammed into the two-story building, said Abu Sayed Mohammad Waliullah, a teacher at the school in Bangladesh’s capital, Dhaka, who had been chatting with other teachers outside. The plane exploded with a bang so loud it sounded like a bomb had gone off, he said.

“My mind went blank. I couldn’t understand what had just happened,” Mr. Waliullah said. Then, after what seemed like 10 seconds, he heard another blast as the plane’s jet fuel ignited.

“This time, it created a big, big fire,” Mr. Waliullah recalled. “I saw