The shooting occurred at the Linderud School in the Grorud area of east Oslo, which is both a primary (barneskole) and secondary school (ungdomsskole) with around 480 pupils and 100 staff.
Pupils and staff were on site at the time and were kept in the school whilst police carried out their investigations.
They were allowed to leave the building just after 5pm, public broadcaster NRK reported.
Police said four people were involved in the shooting, three perpetrators and one victim.
One person had been arrested by police, and police had detained two people before later releasing them.
The police added they were still looking for any victim.
Police said it first received reports of shots fired at 3.43pm.
Witnesses told the newspaper VG that three people were seen running in one direction following the shooting, with a fourth possibly injured person running in a different direction.
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