The girl, who cannot be named for legal reasons, was taking part in the scheme at a Wilko store in Nuneaton when she started the blaze.
According to reports, she was tired of staking shelves and decided to cause enough disruption to be sent home.
It is thought that the fire caused £1million worth of damage to the store.
Warwick Crown Court heard that it took firefighters half an hour to get the blaze under control.
John Mohammed, for the defence, said: "To a degree she got a bit bored, and she would have liked to have done something more exciting than stocking shelves.
"When she went to the goods inward area she intended to start what she thought would be a small fire in a crate, as she described it, which contained cardboard boxes.
"She lit the corner of a box, and did not expect it to spread any further than the cage it was in. She thought it would cause some disruption, sufficient enough for her to be able to go home.
"She acknowledged it was a stupid thing to do. She didn't realise how quickly the fire would spread to other boxes."
The teenager pleaded guilty to the offence on Wednesday and was due to be sentenced, but the judge has ordered for her to be examined by a psychiatrist first.
Judge Alan Parker said: "I am very concerned about her, and very conceded about the dissociation referred to. The fact that she is an intelligent girl makes it more disturbing."
The case has been adjourned pending the psychiatric report.