The investigation also found Mr Mmata encouraged students to drive as part of their "Kenyan experience" and three students drove vehicles on the trip despite this having been banned two years before.
As well, the van was overloaded and some passengers were not buckled in and team leader Philip Russell was asleep in the front seat when Mr Fellows and driver Mr Mmata switched places minutes before the crash.
The investigation subcommittee said it was "of concern" that there appeared to have been a progression in "driving incidents".
"This progression demonstrates an increasing confidence by the students in the acceptability of student driving, a result of Christopher's encouragement, and a mistaken belief by some of the leaders that driving by students was acceptable."
Students would be strictly banned from driving on future trips.
Despite Mr Fellows' admission, Kenyan police had closed the matter after concluding Mr Mmata was the driver.
Mr Hollister-Jones said no disciplinary action would be taken against tour group leaders.