They took a ferry to France, with her using Forrest's wife's passport, sparking a search on both sides of the Channel. Forrest was arrested in Bordeaux, southwest France, eight days later.

Prosecutor Richard Barton said on Tuesday the teacher had committed a "gross and long-term breach of trust".

"This is not This is a 15-year-old girl with her own vulnerabilities, and a 30-year-old teacher," he told the court.

"When parents send their children to school, they quite properly expect that those who teach their children will care for them properly."

Barton said the girl had gone willingly to France, but Forrest could not use this as a defence.

In a video interview recorded on October 3 that was played to the court, the teenager said that in the months before she and Forrest fled, they exchanged flirtatious texts and Twitter messages and eventually started meeting up for sex. The age of sexual consent in Britain is 16.

"I had had boyfriends and crushes on other people but he was older," she said. "I liked that he was older and mature.

"I didn't really like school, talking to him became a reason to start going and made me feel better about things."

The court heard that the night before they ran away, a police officer and a social worker had visited the girl's home and asked questions about the relationship.

She denied having improper relations with Forrest but her mother gave police permission to seize her mobile phone. Knowing that the messages and pictures on the phone would reveal the truth, she packed a bag.

Once in France, she and Forrest dyed their hair in an attempt to disguise themselves and began looking for work, but an English bar owner in Bordeaux recognised them from media reports and Forrest was arrested.

He had been married for a year but relations with his wife were strained, jurors were told.

The trial is due to last for two weeks.