The incident reportedly happened at the end of the school day on Wednesday. The two older boys punched and kicked the younger pupil in the face. Police are treating the incident as severe assault.
"There are still no known motives, but it is very unusual for such young boys to be involved in severe assaults like this," said local police officer Håkan Johansson.
The 11-year-old boy is receiving treatment in hospital and the 12-year-olds have been taken into care. Once the police investigation is completed, social services will take over the case since all the boys are under the age of discretion.