"Staff intervened immediately and the boy who was injured was attended to at the school and then taken to hospital by his parents," Mr Bruce said.
"The boy who had used the knife was immediately isolated from the playground and his parents notified to come and collect him from the school."
The 10-year-old was taken to Middlemore Hospital, where he was given three stitches.
The Counties Manukau police are now investigating.
"The incident was reported to police last Thursday evening, and not by the school," a spokeswoman said.
Mr Bruce said the board of trustees was notified immediately and the Year 4 boy stood down from the school.
Ministry of Education regional manager Bruce Adin said schools were responsible for managing student behaviour "and in instances where a serious assault is alleged, schools should consider calling the police".
Mr Bruce initially refused to comment on the stabbing, but yesterday he relented and said steps had been taken since the incident including: communicating with police, referring both boys and their families to a social worker, and notifying the ministry's rapid response team.
"Meetings were held with the parents of both boys involved. The two boys met and apologised to each other for their actions," Mr Bruce said.
"A staff meeting has been held to outline the incident and to revisit playground duty expectations ...
"Support has been put in place for both boys and also for the wider school population to ensure the general safety of all students, including planned classroom discussions about keeping safe."