Four children were among the dead after two gunmen opened fire inside the school near Sao Paulo.
Another 17 people - mostly school kids - were shot and injured, and several of them were in serious condition, said police, who were not yet aware of a motive for the violence.
"The big question is: what was the motivation of these former students?" de Castro said.
"In 34 years as a policeman, it's the first time I see someone use a crossbow like that", police Col. Marcelo Salles said.
Students gathered outside the school recounted harrowing attacks and seeing several bodies lying in pools of blood.
The unidentified gunmen, who appeared to be between 20 and 25 years of age, shot and killed a worker at a nearby vehicle wash before their attack at the Raul Brasil school, police said.
"My friend was stabbed in the shoulder and my other friend was shot", she said.
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Police officers guard the entrance of the school.
The attackers appeared to be in their early to mid-20s, and authorities don't believe they were former students, the governor said.
Horacio Pereira Nunes, a retiree whose house is next to the school, said he began hearing shots around 10 a.m.
A soldier of the military police battalion and his dog take part in an instructional exercise with officers of an elite unit of the French police, who is responsible for anti-terrorist actions in France, at Terminal 1 of the Tom Jobim International Airport in Rio de Janeiro, Brazil, May 25, 2016.
School shootings are rare in Brazil, although the country is one of the world's most violent, with more annual homicides than any other.
Brazil's new President Jair Bolsonaro recently announced gun ownership controls would be loosened. Soon after being inaugurated January 1, Bolsonaro issued a decree making it easier for people wanting a gun to buy and possess one.