The students were traveling from Western Kenya to Nairobi when the accident occurred.
Witnesses and police said the six died on the spot and that the accident involved a lorry and a matatu that was carrying the students.
Officials said about 10 others who were injured were rushed to the Naivasha county hospital for treatment.
The incident came hours after another accident killed four people in the Molo area on the highway on Monday evening.
Police said a bus veered off the road and landed in a ditch at Twin.
Police said the driver of the bus who escaped with minor injuries told them he was avoiding colliding with an oncoming lorry when he lost control of the vehicle and knocked some students who were walking home from school.
Molo Sub-County Hospital Medical Superintendent Dr. Susan Wanjiku said 17 people were receiving treatment at the hospital in the Monday incident.
Two other victims were transferred to Nakuru PGH Hospital.
Rift Valley police boss Tom Odera said they are investigating the incidents.
Fatal accidents have been on the rise.
At the weekend, 13 mourners were killed in an accident in Wundanyi, Taita Taveta.
Police blamed speeding for the Saturday night incident.
