The crime took place at Blåsboskolan in Västerås.
The victim was bleeding severely but, according to the police, is not life-threateningly injured.
Three people were found near the crime scene and are currently arrested, initially suspected of attempted robbery.
– If there is a robbery or something else behind it, we have to look for interviews with the arrested and with the plaintiff. The crime classification can be changed, according to the police.
All involved are under 18 years of age.
The police are looking for witnesses who were at the scene.