The vehicle reportedly ran into them yesterday afternoon.
Several others were rushed to the military hospital in General David Ejoor Barracks, Effurun, and Central Hospital, Warri.
It was gathered that 12 pupils were rushed to Central Hospital, Warri, among whom nine were confirmed dead on arrival.
The other three were reportedly referred to Delta State University Teaching Hospital (DELSUTH), Oghara, after being stabilised.
The students were said to be returning home after school hours when the incident occurred.
The Nation learnt that three of the deceased pupils were siblings.
A combined team of the army, Federal Road Safety Corps (FRSC) and other security agencies made efforts to recover other pupils believed to be trapped under the tanker.
Firefighters were equally drafted to the scene to prevent fire outbreak.
The police command, through its spokesman Bright Edafe confirmed the accident.
He added that the whereabouts of the driver was unknown.