The attacker, a 15-year-old former student of the school, entered the building at about 9 a.m. and wounded three school employees with a sharp object, police said.
The victims included the janitor, who died despite efforts to resuscitate her, and the school principal, who helped restrain the teenager, broadcaster TVP Info reported.
Interior Minister Marcin Kierwiński confirmed that police had subdued and detained the 15-year-old.
Police officers arrived at the scene within minutes of receiving the call, reporters were told.
The exact circumstances of the incident were under investigation.
Earlier on Thursday, authorities said police had detained a 17-year-old in the central town of Sierpc who planned to carry out an attack at a school and had two knives in his backpack.
On Wednesday, a 19-year-old student attacked schoolmates and teachers with a knife at a vocational school in the northeastern Polish city of Ostrołęka, injuring nine people before fleeing the scene, authorities said.
Prosecutors said police later found a homemade explosive device in his backpack that he had attempted to detonate at the school.
(gs)
Source: IAR/PAP