Four employees at the Brynseng School in east Oslo were taken to a local hospitals with minor injuries, Oslo police told Norwegian media agency NTB.
Police were called before 9am local time following reports of a student threatening a teacher with a knife.
The boy was arrested but police have not found an immediate motive, police stated. Officers have launched an investigation and were speaking to witnesses, Sven Christian Lie of the Oslo Police district said in a press release.
"It is a very young boy who is arrested. I can't say anything more about the grade level. He was arrested inside the school," Mr. Lie said.
A press statement said: "Four people were taken to the emergency care department for treatment.
"The injured are employees of the school. We are still on the site with several patrols and are conducting witness interrogation."
Brynseng school opened in 2017 and has over 800 enrolled pupils. The school declined to comment to Norwegian media despite requests to do so.
But local media quoted a text message sent by headteacher Maria Tindberg, stating to parents that the situation was "calm and under control".
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According to a survey for Norway's educational association last year, one in five teachers had been the victim of violence in the last 12 months.