President Rousseff later said she was shocked by the massacre.
“Innocent children lost their lives and their future,” she said, after observing a minute of silence for the victims.
Rodrigo Alves Pereira, who lives in the area, told the BBC that he was in a shop near the school when he heard “a lot of shots coming from inside the school”.
“I went out and saw many children… coming out of the school. Some of them were covered in blood, shaken, asking for help. One was injured in the shoulder. They were desperate and crying.
“They said that there was a man wearing a suit who was shooting at school kids in classrooms. He first went to a reading room, talked to teachers there, and then went on to two classrooms. This is a complete tragedy. We are all devastated,” Mr Pereira said.
Marcos Silva, 11, who was at the school but was not hurt, said the experience was “like a horror movie”.
“Everyone lay on the ground in silence, the teacher asked us not to make a sound so he would not notice us,” he added.
“I thought to myself: ‘If he comes in, we are all going to die’.”
Rescue workers used a football pitch near the school as a helicopter base from which to transport wounded children to the hospital.
Hundreds of people gathered outside the school, either out of curiosity or to check on children who were inside at the time of the shooting, the BBC’s Paulo Cabral in Rio de Janeiro says.
The school, in an impoverished neighbourhood of Rio, is attended by pupils aged nine to 14.
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