The 76 schoolchildren and their teachers were saved just in time when the driver reacted quickly to the smell of burning as the bus drove along the A7 motorway near Itzehoe in north Germany.
The driver had been alerted to the danger after detecting a smell of something burning coming from the engine, the fire service said.
"We pulled over quickly, I shouted ‘everybody out of the bus’,” Joachim Roßdeutsche told NDR broadcaster on Monday.
The teenagers, aged between 13 and 14, left the double-decker vehicle, piling out onto the roadside at the Harburg rest stop area.
Shortly afterwards, the interior of the bus burst into flames.
When the fire service arrived at the scene, the whole of the back part of the bus was burning and the windows had shattered, NDR said.
One pupil was sent to hospital with shock, said police. By Monday morning the cause of the fire had not yet been determined.
The pupils from Itzehoe were heading to Oxford for a school trip. Most of their luggage was saved from the flames.
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