The woman, who is employed at the school, was beaten with a machete and had to be taken to hospital.
The student, who is under 15 years old, was found at the scene.
- Since he is under 15 years old, we are investigating the incident in consultation with guardians and social authorities, says Richard Lundqvist, spokesman for the police.
Parts of the school were cordoned off and examined during the evening by forensic technicians.
The incident, which is classified as attempted murder, is said to have taken place inside the school at 15:30.
The woman's injury situation is currently unknown.