Azimio leader Raila Odinga has condoled with Pwani University fraternity following an accident along the Nairobi-Nakuru highway on Thursday.
The accident which involved the school bus and a Nissan matatu in Kayole-Karai area in Naivasha claimed at least 12 lives.
Some are feared to have been teachers and students.
"I extend my sincere condolences to the Pwani University community and all those who lost colleagues, friends, and loved ones in the regrettable accident that involved a Pwani University bus and matatu on the Naivasha-Nakuru highway," Raila said in a tweet.
The accident is said to have occurred after a suspected break failure.
Rift Valley police boss Tom Odera said the death toll may rise.
"The bus rolled and killed 12 on the spot. We may have more casualties," he said.
Odera said the bus had about 30 people including teachers and students who were heading to a sports competition.
He added that the driver of the bus lost control and hit at least five vehicles before rolling into a ditch.
A number of injured persons were taken to nearby hospitals for immediate medical attention.