"It's a very sad scene - the saddest thing I've seen in my entire life", said Sao Paulo Governor João Doria.
Salles added the gunmen, who police said appeared to be between the ages of 20 and 25 years old, used at least one.38-calibre pistol, along with homemade bombs and a crossbow.
Horacio Pereira Nunes, a retiree whose house is next to the school, said he began hearing shots around 10 a.m.
Instead of face police, they turned their weapons on themselves, authorities said.
Kelly Milene Guerra Cardoso, 16, said she and other students took refuge in the school's cafeteria, locked the door and lay on the floor.
Marcelo Salles, commander of police forces in Sao Paulo state, spoke just outside the school and said that in his over three decades of service, he had "never seen anything like this, it was an unspeakably brutal crime".
"I heard a lot of screaming", she said.
Teacher Sandra Perez told O Estado de S.Paulo newspaper: "I was in the classroom during our break".
A shooting at a school near Sao Paulo on Wednesday left several people dead, police sources said, with initial reports putting the death toll as high as eight.
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.
Far-right President Jair Bolsonaro made relaxing gun control a cornerstone of his campaign past year.
The names and identities of the victims and the attacks have yet to be released and the motive for the attack is not clear.