The first that Eliene Almeida, head teacher at the municipal school in Bento Rodrigues, knew of the deadly mud flow that destroyed her village was a cry from her husband.

Most villagers were running for higher ground after hearing a dam at the local Samarco mine had burst, but no one inside the packed school was aware that a 20 meter-high wall of mud and water was approaching.

Almeida's husband raced to the school and sounded the warning.

"He came in shouting that we had to run," Almeida, 31, told Reuters at a hotel housing the village's survivors.

Frantically, she rounded up the children, aged mainly between 11 and 16. "Within three minutes everyone was out."

The flood killed at least four people and on Monday, four days after the disaster struck, 25 people were still missing. But Almeida's 58 students all survived.

Wearing red leggings and a purple T-shirt she cradled her 18-month-year old son as she recalled the evacuation calmly on Sunday in the playground of the hotel.

An aerial view of the extent of the damage caused to the village (AFP)

Her toddler's foot was in a cast after taking a fall in the hotel. "He's getting used to his new home," she said, managing half a smile.

There is little left to see of the school that was a pride of the village of 600 people. Only the roofs are visible, the rest submerged in thick sludge of water and iron ore waste from the dam at Samarco, owned by mining giants Vale SA and BHP Billiton.

The lack of a warning siren or an emergency plan for evacuating villages near the dams is a constant complaint of those hit by the floods and something prosecutors say they will pursue.

A 2013 report commissioned by a state prosecutor warned of serious safety problems with the Samarco dam. It said an emergency plan should be set up for Bento Rodrigues, with practice drills, as conditions for renewing a license for the dam. Residents say no such plan was ever formulated.

Debris of a house in the Bento Rodrigues district (Reuters)

The mayor of the nearby town Mariana, Duarte Junior, himself admitted to hospital on Sunday with a feared heart attack due to a lack of sleep and stress since the accident, called Almeida a "hero."

"I don't see it like that," she said with a shrug. "Anyone would have done the same."

She said it was lucky the flood hit in the afternoon, when older students, who could move more quickly, were in class.

Another factor was the wide doors at the entrance that allowed people to escape. "It could have been so much worse."

Almeida hopes to open a new school and says it is important children resume their lessons. The local government appears supportive but, she says, things will never be quite the same.

"You can build a new school, but all the work that went into that school in Bento, what it meant to the village, that's gone forever.