Iran Press/ Europe: The suspected attacker, also aged 12, has been detained and taken into custody. Police also said they had the weapon.
Officers were called to the incident at Viertola School in Vantaa just after 9 am local time on Tuesday.
Two other pupils aged 12 were also shot and seriously wounded, police chief Ilkka Koskimaki told reporters and were taken to hospital.
The victims and the suspect were apparently from the same class, Finnish broadcaster MTV Uutiset reported.
Emergency services - including armed police officers - responded.
Some of the children reportedly hid during the attack, while others who had been contacted by their parents on mobile phones said they saw what happened.
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