A heroic teacher saved the lives of more than 1,600 children in the Nepal floods after their school was washed away by a landslide. Rajendra Dawadi, the principal of Tribhuvan Trishuli secondary school in the Trishuli valley of north-central Nepal, received a phone call from his friend on Wednesday morning telling him the village five miles upstream had been taken by the river. Nine hundred children had already arrived at the school, while 700 more were on their way on foot and by bus. He was then warned by a parent that the water was rising fast towards the school, which stood 200ft above the river. It came amid devastating landslides and flash floods which struck Nepal on Wednesday, leaving 734 people dead and 2,498 missing. Authorities warned of possible fresh flooding today and urged residents in affected districts to remain on high alert. Two teenagers, a 13-year-old girl and a 14-year-old boy, are among 33 British nationals believed to be unaccounted for. Knowing his school was in trouble, Mr Dawadi, 47, quickly began calling the drivers of the buses on their way – managing to get through to all but one – and ordered them to turn back. Rajendra Dawadi, the principal of Tribhuvan Trishuli secondary school in the Trishuli valley of north-central Nepal, saved the lives of more than 1,600 children in the floods An aerial view of the destruction along the Trishuli River corridor. The school is circled in red What the school used to look like, before it was completely destroyed in the Nepal floods The water had already started entering the school grounds when he spotted the last bus, which he told to turn around. 'It got to the other side just as the bridge collapsed,' he told The Times. He then turned his focus to the children inside the school, who had realised they were in danger.'One girl had a panic attack, so we put her on the back of a motorbike. The rest I sent uphill on foot, then followed,' the teacher said. The terrified children ran up the hill, where they took shelter under a bodhi tree. Mr Dawadi's quick thinking saved 1,643 children and all but one member of staff. Santos Pariyar, 32, a social history teacher who was new at the school, had come in early that day to teach extra classes and was washed away by the floods. More than 3,700 people have been rescued in Nepal so far. Last night, the Nepali army inserted a pipe into a tunnel at the Trishuli 3A hydroelectric site where workers are understood to be trapped.The home minister, Sudan Gurung, said: 'We have already started sending air through the small opening, and we will now begin excavation work and send the rescue team inside.' More than 100 workers could still be alive inside the tunnels, with rescue efforts focused on the Trishuli 3A and 3B sites.The chunk of ice and rock said to have snapped off and triggered the flash floods had an estimated area of nearly 50 acres, according to Daniel Shugar, a geomorphologist at the University of Calgary.It plunged 1,200 metres down the north face of Langtang Lirung peak into the upper gorge of the Lhende Khola river, near the Tibetan border. Authorities in Nepal warned of possible fresh flooding today after water levels rose in the Bhotekoshi River.Locals were sent flood warning notices on their phones this morning as rescue crews worked to clear a major highway connecting Kathmandu to one of the areas worst hit by Wednesday's catastrophic floods.In the town of Galchhi, a group of residents sought higher ground on a nearby bridge and watched as the river swept through. The combined toll in Nepal and Tibet rose to 750 people dead and more than 3,000 missing after the latest updates today. Nepal's disaster agency said the death toll had climbed to 734 with 2,498 people missing, while Chinese authorities reported 16 deaths and 546 missing, including more than 100 Nepalis and people from over 12 countries.The latest warnings came after Chinese authorities informed their Nepalese counterparts of increased water flow upstream. But residents of the flood-hit villages said they were struggling to make sense of near-constant alarms they have received since the initial floods.Resident Govind Shreshta said: 'Sometimes they say a flood is coming, then we have to run again to higher ground, even at night. There is also rain.'It's really difficult for me. I have a 72-year-old father and 70-year-old mom. I must carry them and I am exhausted.' Damaged homes in Trishuli Bazar following the flash flood, in Nuwakot district, Nepal Mr Dawadi is pictured, right, outside the school before tragedy struck Devi Ghat town was torn to shreds by the floods, with livestock left rotting on top of the mud Devi Ghat was devastated by the floods which killed hundreds of people and left thousands missing Dramatic footage showed the moment hikers witnessed a giant glacier collapse in the Himalayas The video, filmed by a Chinese tour guide in Tibet, shows huge chunks of ice breaking away from a ridge on Langtang Lirung peak, on the Nepali side of the borderLarge concrete and smaller suspension bridges were destroyed, leaving communities on the other side of the river with less easy access.Prithvi Highway is a crucial supply route for food, fuel and other goods entering Kathmandu from other parts of the country. Malaysia's Prime Minister Anwar Ibrahim said today that his country will send a specialized disaster search-and-rescue team to Nepal to assist with relief efforts and help evacuate Malaysians affected by the disaster. Malaysia has also pledged $1million in financial aid to Nepal. Last night, dramatic footage showed the moment hikers witnessed the giant glacier collapse in the Himalayas, triggering the catastrophic flash floods and mudslides. The video, filmed by a Chinese tour guide in Tibet, shows huge chunks of ice breaking away from a ridge on Langtang Lirung peak, on the Nepali side of the border.Hikers reported hearing a huge crack before ice, rock and debris plunged into the valley below, sending an enormous cloud of dust billowing into the air.The glacier collapse triggered a landslide which formed a lake in the valley below, before a torrent of water and debris surged through the mountain river system towards the Gyirong border crossing.A second video shows debris racing down the mountainside immediately in the aftermath of the collapse, marking the beginning of the disaster that would devastate communities downstream.