The children reportedly picked up a grenade, which may be linked to unrest in Burundi.
At least six children have been killed and 25 others injured in an explosion on Wednesday at a primary school in north-west Tanzania, witnesses say.
Reports say the pupils were playing with a grenade they had found.
Three were pronounced dead at the time they arrived at hospital. The others succumbed to their wounds, Rulenge hospital's doctor in charge, Sister Maria Goreth Fredricks, said.
The school is in Kagera region near the border with Burundi.
The BBC's Leonard Mubali in Tanzania reports that the region hosts refugees from neighbouring Burundi, some of them ex-soldiers, who could have abandoned their weapons in the area.
He says that the device that exploded is likely to be a grenade.
Kagera regional police chief Augustine Orome said a security team had been dispatched to the school in Kihinga village.The reporter adds that the region is also a transit point for arms and it is not uncommon for residents to come across weapons.