Police said the 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.
Two other victims were transferred to Nakuru PGH Hospital.
Rift Valley police boss Tom Odera said they are investigating the incident. He added the survivors were taken to the hospital.
The wreckage was towed to Mausummit Police Station.