The serious assault happened on Thursday 28 January in an alleyway close to Poole High School off Wimborne Road in Poole.
The victim was taken to hospital for treatment to a single stab wound. His condition is not life-threatening and no one else was injured in the incident.
Officers from the Neighbourhood Policing Team are carrying out high-visibility patrols to reassure the community.
The incident is not connected to the reported stabbing in Poole Park on Saturday 23 January.
If anyone has any information that may help officers with their investigation, they are asked to call Dorset Police on 101, quoting incident number 28:260.