A bus passenger woke up surrounded by smoke as the driver screamed after a teenager set light to one of the seats.
Logan Pritchard, then 17, was "messing around" with the lighter after boarding the 10A Stagecoach bus on Bridge Street, St Helens.
He got off around 20 minutes later but had lit one of the seats on the top deck, which shortly began to fill with smoke as a college student slept at the back of the bus.
It was only when a new passenger went upstairs and saw the smoke that the driver was alerted and everyone evacuated.
Mike Stephenson, prosecuting today at Liverpool Crown Court, explained Pritchard went to the top deck of the bus after boarding it on January 28.
He was followed by a college student who "went to the very back and sat on the top deck feeling unwell and had been sent home from college".
Mr Stephenson said the student spent "much of the time there with his eyes shut and recalls hearing repeated clicking which he associated with a cigarette lighter".
Pritchard, now 18, can be seen leaving the bus 18 minutes after getting on after which a "plume of smoke" develops.
When a new passenger got on the bus and went upstairs they "saw the smoke and alerted Julie Friend, the driver".
She stopped the bus and evacuated it, at which stage the student had fallen asleep on the back seat and was "woken by a crackling sound and the cries of the driver".
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After a police appeal to find Pritchard in relation to the arson attack his girlfriend phoned police and he attended an interview in which he accepted he had a lighter but denied starting the fire.
The court heard Stagecoach lost £30,000 through £16,325 of damage and loss of use of the bus.
Mr Stephenson explained the student suffered a sore chest and was "anxious".
Judge Robert Trevor-Jones said: "He only woke, possibly by a combination of the smoke but by the cries of the driver."
Jeremy Hawthorne, defending, said: "This is very difficult for the defendant to come to terms with, he accepts this is a very serious matter."
He explained Pritchard had left the bus before a fire broke out and said he did not realise he had set fire to the seat.
Mr Hawthorne said he was a "teenager messing around" and said there was no history of mental health and "no animosity towards the bus company or anybody else".
He added: "Teenagers mess around and on occasion something happens."
Mr Hawthorne explained that he had been working with his mum at a pub in Warrington.
He said: "Despite looking down the barrel of this most serious matter he voluntarily contacted police."
Mr Hawthorne said: "I wouldn't want anyone to think he wasn't most thoroughly regretful.
"What started out as messing around has led to a level of damage and a level of danger that he had no actual comprehension of at the time."
Pritchard, of Hillside Avenue, Denton Green, St Helens, has no previous convictions.
He admitted arson being reckless as to whether life was endangered
Judge Trevor-Jones, sentencing, said it was "stupid" and "reckless".
The judge said: "It is a really classic indication of the dangers which exist when people mess around with fire."
Judge Trevor-Jones said: "I accept when you left there was no real indication a fire would break out in the way it did but clearly it did, there was always that risk."
The judge added that both decks of a bus can be destroyed by a fire in seven minutes.
Pritchard appeared stunned as he was told he would be jailed for two years and nine months and ordered to leave with the dock officer.
