The suspects are believed to be two teenagers, who shot themselves after the rampage.
She said the attackers had guns and knives, and when she ran to the principal's office, she saw many dead people.
Doria said the dead included two teachers and six students, and several more people had been hospitalized after sustaining injuries.
Police work at the scene of a shooting at a school in Suzano, Sao Paulo metropolitan region, Brazil, on March 13, 2019.
Doria said the school had been evacuated and police were inspecting what appeared to be possible explosives left by the shooters.
At least 17 other people, mostly children, were also shot at the Raul Brasil elementary school and taken to hospitals, police said in a written statement.
Teacher Sandra Perez told O Estado de S.Paulo newspaper: "I was in the classroom during our break". When I realised they were gunshots, I stayed there.
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School shootings are rare in Brazil, even though the country is one of the world's most violent, with more annual homicides than any other.
The men, identified as former students at the school in a suburb of Sao Paulo, also shot and killed the owner of a used auto business nearby before launching the attack on the school, authorities said.
The last one occurred in 2011, when the former student of a school returned and shot 12 children fatally, Fox News reports.
Another shooting that took place about 500 metres from the school happened before the killings, but it was not yet clear if the two incidents were linked.
Brazil's new President Jair Bolsonaro recently announced that gun ownership controls would be loosened.
Gun laws are strict in Brazil, but it is not hard to purchase a weapon illegally.