US President George W Bush has visited the school in Enterprise, Alabama, where eight pupils died in a tornado two days ago.
The president clambered through the textbook-strewn rubble, and declared the district a disaster area.
He described it as "a miracle" that 100 children escaped the building alive.
At least 20 people died on Thursday as fierce storms and as many as 30 tornadoes pummelled states across America's south and mid-west.
The tornadoes were part of a major storm system that stretched across a swathe of the United States.
Mr Bush flew over some of the other states hit by the violent weather and promised help to those who lost relatives and property.
"I want the folks at Enterprise to know that, to the extent that our government can... working with the state, we will help rebuild the school system, this high school," he said.
Quick reaction
The disaster declaration means that residents can apply for federal relief funds for home repairs and temporary housing.
The Federal Emergency Management Agency (Fema) said equipment and supplies of food and water had been moved quickly into the area.
Both the agency and President Bush himself were strongly criticised for responding slowly to Hurricane Katrina in 2005.
Fema's director David Paulison said the agency had been quick to respond to the latest disaster. "That's the new Fema," he said.
How a tornado is formed
"The system we used in the past, waiting for a local community to become overwhelmed before the state steps in, and waiting for the state to become overwhelmed before the federal government steps in, doesn't work."
The storms killed 10 people in Alabama, nine in Georgia, and one in Missouri.
They came a month after 20 people were killed by a tornado in Florida.
Mr Bush walked in private through the hallway where the 16 and 17-year-old students were crushed to death, after school administrators took the decision that they would be safer inside than if ordered to leave the school as the storms approached.
He met some of the survivors.
"Today I have walked through devastation that is hard to describe," he said.
"One hundred kids got out of here alive, which is a miracle.
"We can never replace lives and we can't heal hearts except through prayer, and I want the students to know and the families to know that there are a lot of people praying for them."
TORNADOES REPORTED IN THE US
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