A jury found 30-year-old Forrest guilty over the September episode in which the pair spent seven days on the run, sparking an international manhunt.
As the jury returned to deliver its verdict Thursday after a two-week trial, Forrest told the girl "I love you".
She burst into tears as the guilty verdict was announced, and told him "I'm sorry" as he was led away to the cells.
Forrest's defence lawyer had argued he felt he had to run away with the girl because he feared she would commit suicide.
The jury at Lewes Crown Court in East Sussex, southeast England, had heard that the pair absconded on September 20 last year, a day after police questioned the girl about the relationship.
They took a ferry to France – with the teenager using Forrest's wife's passport – sparking an intensive search on both sides of the Channel.
Forrest was arrested in the port city of Bordeaux, southwest France, eight days later.
Prosecutor Richard Barton said the teacher had committed a "gross and long-term breach of trust".
"This is not Romeo and Juliet – 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 3rd 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 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.
"I wanted to leave because I didn't want to face my mum when she found out," the girl said.
Once in France, she and Forrest dyed their hair in an attempt to disguise themselves and began looking for work using false CVs, but an English bar owner in Bordeaux recognised them from media reports and Forrest was arrested in the street.
He had been married for a year but relations with his wife were strained, jurors were told.