The shooting took place at about 12:45 p.m. local time (19:45 GMT) near a building complex on Fountain Street that houses several schools, including Rudsdale Newcomer and Bay Area Technology School, and Sojourner Truth Independent Study school, local media reported.
Lt. Ray Kelly of the sheriff's department told reporters that first responders had arrived on the scene at about 1 p.m. local time, adding that the shooting was "probably a targeted event."
Victims of the incident were taken to hospitals while students were evacuated from schools after a brief lockdown. Three of the six injured were said to be in critical condition. It remains unclear whether all six were students, but officials have detailed that they were "adults."
KION-TV News Channel reported that police said there were three shooters involved in the incident, who are not believed to still be at the school. Kelly has explained that they fled the scene.
"Today's gun violence at Sojourner Truth school shocks the soul -- our schools are sanctuaries for our children," Oakland Mayor Libby Schaaf said in a tweet. "Our investigators report all six victims are adults + being treated for injuries at hospitals now. The school is now clear + all children being reunited w/ families."
An investigation into the incident is ongoing as officials work to determine what prompted the shooting.