Wednesday morning at Tribhuvan Trishuli School in Nuwakot, north of Kathmandu, Principal Rajendra Dawadi received a warning about an approaching flood around 9:10 a.m. With about 900 of the school's 1,643 students present — only older pupils ages 11 to 18, as younger children started later — Mr. Dawadi ordered an immediate evacuation to higher ground. Students who arrived by bus were sent back on them, while traumatized children were transported on teachers' motorbikes. Within 13 minutes, by 9:23 a.m., a deluge of water, mud and debris destroyed the school. At least one teacher remained missing as of Monday.

The disaster has killed nearly 1,000 people in Nepal, with at least 4,200 missing. In Bharatpur, southwest of the school, hundreds of bodies washed downstream, overwhelming local morgues. Rescue workers struggled to reach workers trapped in hydropower tunnels, and thermal drones detected no signs of life. Roads and communications were severely disrupted.

"I am extremely emotional to see the school completely swept away," said Mr. Dawadi, a former student there. Only two school buses remain. "We saved lives, but now there is no place to teach the students," he said, expressing worry about resuming education.