Police said they had arrested one man after the incident at Brinell School in Fagersta, central Sweden.
Officers were called to the scene just after 14:00 local time (13:00 BST) on Friday, with some reports in Swedish media saying the suspect was shot in the leg by police.
Fagersta council did not specify the number of wounded, but said police were searching the building and that eight other schools had been placed under lockdown measures.
Police have said there is currently no danger to the public.
In a post on X, Sweden's Prime Minsiter Ulf Kristersson described the attack as a "serious incident," and said "we do not know what lies behind the act, but we know that police are working very intensively".