The shooting took place at the Viertola school in Vantaa, a suburb of the Finnish capital, with around 800 students and 90 staff.
A 12-year-old student opened fire at a secondary school in southern Finland and wounded three other students on Tuesday, police said.
Heavily armed police cordoned off the lower secondary school, with some 800 students, in the city of Vantaa, just outside the capital, Helsinki, after receiving a call about a shooting incident at 09:08 am.
Police said both the suspect and the wounded were 12 years old.
The Finnish newspaper Ilta-Sanomat said the suspect was arrested in the Helsinki area later on Tuesday.
Authorities asked people in the area to stay away from the school and to remain indoors while not letting in strangers.
The school, housed in two locations, also includes Swedish immersion groups, special education classes, and outpatient education.