Officers say a boy aged eight and two nine-year-old girls were hurt when a pupil - also aged nine - fired pellets from a ball bearing gun at Storrington Primary School.
They say the boy handed over the gun to school staff after the shooting.
Police say there was ‘no evidence of any criminal intent’ and that they have spoken to the parents of the boy who fired the gun.
In a statement today, Sussex Police say: “Officers have investigated a report that a ball bearing gun was discharged at Storrington primary school on Thursday (28 March).
“Three pupils, an eight-year-old boy and two nine-year-old girls accidentally sustained minor bruising when the gun was fired by a nine-year-old boy who had brought the gun to school from home.
“It was not fired at anyone and there is no evidence of any criminal intent.
“Three pupils were injured when pellets are believed to have ricocheted off walls and doors.
“Staff had been told what was happening and the gun was handed to them by the boy.
“We have talked to the staff and the families of the children who were hurt to ensure that they are fully aware of what has happened.
“The family of the boy have been seen and advised about the safe keeping of BB guns.”