Five children are reportedly among the dead, as well as a school employee and a passerby.
It was a horrifying scenes in Sao Paulo, Brazil and the adult who was killed was the principal of the Raul Brasil public Grade school.
Police work at the scene of a shooting at a school in Suzano, Sao Paulo metropolitan region, Brazil, on March 13, 2019.
Website UO1 said the two killers, "probably teenagers, entered the school shooting with their heads covered by a hood".
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.
However, current Brazilian President Jair Bolsonaro signed a temporary decree in January that made it easier for citizens to buy guns in an effort to overturn the strict regulations of the past.
The attackers were trying to force their way inside a room at the back of the school where many students were hiding when police arrived.
Gun laws are strict in Brazil, but it is not hard to purchase a weapon illegally.
Another ten people, mostly school children, were shot and injured, with several in serious condition, said police.
Doria said the school had been evacuated and police were inspecting possible explosives left by the shooters.
"I heard a lot of screaming", she said.
The last one occurred in 2011, when the former student of a school returned and shot 12 children fatally, Fox News reports.
The wounded student who took shelter in her house could not speak.
Speaking a few blocks from the public school in Suzano, Mr Doria said: "The school is on lockdown".