A school minibus with pupils on board caught fire in Kenton Road, Kenton, on Wednesday.
The bus belonged to Harrow High School and Sports College in Gayton Road, Harrow.
It is believed a woman driving behind beeped and got the bus driver to pull over in Kenton Road, Kenton, after she noticed petrol dripping from the back of the van at 4.30pm.
Student Holly Taylor,18, saw the fire. She said: "I saw a big black cloud of smoke and heard sirens from fire engines.
"I went to have a look and spoke to a woman who was driving behind the bus.
"She said she saw liquid dripping out of the bus and realised it was petrol.
"If the driver had not pulled over someone could have been seriously hurt."