The suspect was detained by officers near the city's beach, the Mazowieckie provincial police department said on X.
"Police officers from Ostrołęka detained a 19-year-old man suspected of attacking people with a sharp object near and on the premises of a school in Ostrołęka," police said.
"Initial information indicates that seven people may have been injured," the post added.
Police said officers were at the scene investigating the circumstances of the attack.
A local fire service spokesman, however, said eight people were among the injured, three adults and five teenagers.
All of the injured were taken to hospitals, according to media reports.
The building of Vocational School Complex No. 4 was evacuated and closed following the attack.
Ostrołęka Mayor Pawel Niewiadomski said he had ordered that no unauthorized people be allowed onto the premises of other schools run by the city.
"The situation has been brought under control, and the perpetrator has been detained," Niewiadomski said, adding that emergency services were working at the scene to ensure safety and establish the circumstances of the incident.
He urged students, parents and school employees to remain calm and follow instructions from school authorities and emergency services.
Niewiadomski said psychological support would be provided to anyone who needed it, with particular attention given to students who may have experienced severe stress or felt threatened.
He also appealed to the public not to publish photos or recordings from the scene on social media and not to spread unverified information.
"The most important thing now is safety and providing the injured with the necessary assistance," he said.
Niewiadomski said the city would continue working with emergency services and take all necessary measures to ensure safety in schools.
"Our thoughts today are primarily with those who were injured, their families, students, teachers and everyone affected by this difficult event," he said, wishing the injured a speedy recovery and the school community "strength and peace."