Schoolchildren from Milton Road Primary School, in Cambridge were on their way to Lackford Lakes in Suffolk, when the trip leader noticed smoke coming from the rear of the coach.
The double decker bus with more than 53 students and 14 adults on board, pulled over on the A1101, near Bury St Edmunds and children and staff were safely evacuated and checked by ambulance staff. Another coach was sent to take the children back to school.
Fire and Rescue said no one was injured during the blaze, but about 300 litres of diesel had been drained from the bus into a ditch and the Environment Agency had been informed.
Witness Sean Hale said: "The tyres exploded in the fire and you could see all the diesel spilling out."
The head teacher, Dr Michael Catchpool rushed to the site and kept the school and anxious parents updated on the event.
He later said in an email that he was "proud of the way the children behaved and [was] grateful for the professional actions the teaching staff and volunteer adults took."
Support will be offered by the school to children who may need it.
