The accident occurred after a pick-up collided with the school bus.
The accident happened at Mukinduri on the Kerugoya-Baricho Road in Kirinyaga County.
All the dead were inside the pick-up and none of the students in the bus were hurt in the accident.
Two of the deceased died on the spot when the pick-up that had seven occupants collided head-on with the bus belonging to Kamuiru Boys High School.
Three other people died while undergoing treatment at Kerugoya Hospital.
According to Kirinyaga county police commander Leah Kithei, initial investigations showed that the pick-up was speeding.
"The pick-up was speeding and tried to overtake. He went into the opposite lane and collided with the oncoming bus," the police boss said.