The attack took place at about 12:30pm local time (10:30 GMT) on Saturday, when the woman used a "dangerous object" to attack a 17-year-old student, according to Warsaw police spokesman Jakub Filipiak. The suspect also injured two other people, aged 17 and 24.
"One person was injured and transported to the hospital with non-life-threatening injuries. Two other bystanders also sustained injuries but did not require hospitalisation," Filipiak said. Poland's state news agency reported that the two additional victims had gone to the school to submit enrolment documents.
Police did not name the institution but said it was open on a Saturday because it is a higher education establishment. National newspaper Wyborcza identified it as TEB Edukacja, a post-secondary school, and reported that the suspect studied there. Initial findings suggest she did not know the victims.
The attack came as police detained 76 people and checked nearly 50,000 schools nationwide in an effort to prevent further violence. It followed two earlier incidents this week: on Wednesday, a 19-year-old student injured nine people at a vocational school in the central Mazovian province, and a day later a 15-year-old entered a primary school in Leszczydol-Nowiny and killed a 37-year-old caretaker, wounding two other adults.
Schools across Poland tightened security on Friday, with some suspending classes, many parents keeping children at home and police patrols deployed to educational institutions. Prime Minister Donald Tusk said ahead of the operation: "When it comes to the safety of Polish children and peace in our schools, there is no room for compromise."
Warsaw police said they could not confirm whether the latest attack was related to the two previous incidents. Local media, citing a police spokesperson, reported that authorities had information suggesting the week's attacks could be related to an internet challenge.