Three students were sucked into the ground by a patch of quicksand in the dried-up Red River bed April 11.
Nguyen Thu Huong, Kim Hong Thu Huyen, both young girls, and Do Quang Thuy, a boy, all of whom were tenth-graders at Ha Hoa High School in the northern province of Phu Tho, were chasing each other on the dry river bed for fun when the accident happened, said Ha Hoa principal Doan Thanh Binh.
Huong and Thuy were caught in the quicksand first and began sinking. When Huyen came to save her friends, she too became stuck and all three children died at around 4 p.m.
Five other school friends with them at the time shouted for help but no one arrived in time.
Huong and Thuy's bodies were found two hours later. Huyen's was found later that night.
Ten students have previously died in several similar quicksand incidents on the Red River in Hanoi.