Five children are reportedly among the dead, as well as a school employee and a passerby.
"It's a very sad scene - the saddest thing I've seen in my entire life", said Sao Paulo Governor João Doria.
Before entering the Raul Brasil school in Suzano, a city on the eastern outskirts of metropolitan Sao Paulo, the assailants, former pupils aged 17 and 25, shot and killed the owner of a auto rental agency from which they had stolen a vehicle.
Two gunmen open fire on students of the Raul Brasil State School in the city of Suzano in Greater São Paulo. He says authorities don't believe the two were former students.
Far-right President Jair Bolsonaro made relaxing gun control a cornerstone of his campaign a year ago.
"When I arrived at the school.it was chaotic".
Gun crime is extremely common in Brazil but school shootings are rare.
Gov. Joao Doria said the two attackers, who entered the building and started shooting around 9:30 a.m. local time, were believed to be between 20 and 25 years old.
The last major school shooting was in 2011, when 12 children were shot dead by a former pupil in Rio de Janeiro.
"I heard a lot of screaming", she said.
The attack was carried out by two "armed and hooded" teenagers, according to police.
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.
"We can't let those who take advantage of this tragedy speak about how disarmament is the solution", he tweeted, adding: "Weak and shameful 'disarmament farce, ' which gave guns to criminals and prevented self-defence".
A man was injured in another shooting near the school earlier, but police said they were still investigating if both cases were connected.
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