The pupils were playing with a grenade they had found.
Three were declared dead at the time they arrived at hospital. The others succumbed to their wounds, said Rulenge hospital's doctor in charge, Sister Maria Goreth Fredricks, according to a BBC report.
The region hosts refugees from neighbouring Burundi, some of them ex-soldiers, who could have abandoned their weapons in the area and 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 region is also a transit point for arms and it is not uncommon for residents to come across weapons.