Married British teacher Jeremy Forrest was on Thursday found guilty of abducting a 15-year-old schoolgirl after they fled together to France.
A jury found 30-year-old Forrest guilty over the episode in which the pair spent seven days on the run, sparking a high-profile international search.
The prosecution had labelled the mathematics teacher a "paedophile" who had "groomed" the vulnerable teenager, who cannot be named for legal reasons.
But the girl said in evidence that she had encouraged the relationship.
"Jeremy Forrest grossly abused the trust placed in him," detective inspector Mark Ling told reporters outside the court.
"It is every parent's worst nightmare to have a child abducted."
As the jury returned to deliver its verdict 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. He is to be sentenced on Friday.
The charge of abduction carries a maximum sentence of seven years. Forrest has been in custody since his arrest in France in September.
Ahead of the verdict, his father Jim collapsed in the court and was taken to hospital.
Forrest met the pupil at a school in southeast England, and they first kissed when she was 14, Lewes Crown Court heard.
They exchanged flirtatious text messages, topless photos of her and publicly visible tweets, then began a sexual relationship shortly after her 15th birthday, although colleagues had repeatedly warned Forrest to keep his distance.
For several weeks Forrest would pick her up in her school uniform for regular trysts in his car, in hotels and at his marital home.
The girl, now 16, said in a police interview: "It got to the point where there was only so much of a friendship you could have before it developed into something else."
The two fled Britain after she was questioned by police, who seized her phone, meaning the affair was bound to be exposed.
With the schoolgirl using Forrest's wife's passport, they boarded a cross-Channel ferry to Calais in France on September 20 last year.
Police launched a highly publicised search as family and friends issued frantic appeals for their return.
The pair dyed their hair, adopted false names and threw away Forrest's mobile phone.
They were finally found a week later as Forrest was trying to find work in Bordeaux, southwest France. A bar owner recognised him and called police.
In a statement read out on her behalf outside the court, the schoolgirl's mother said "every aspect of our lives has been affected to some degree" by the case.
"As a family, for the last nine months it's been like living out your worst nightmare," she said.
Forrest's defence lawyer had argued he felt he had to run away with the girl because she was "desperate and suicidal" and he feared she would harm herself.
The teacher himself, a keen amateur musician, chose not to testify in his own defence.
He had been married for a year at the time of the affair, but relations with his wife were strained, jurors were told.
Prosecutors acknowledged that the girl had gone willingly to France, but this could not be used as a defence.
"I wanted to leave because I didn't want to face my mum when she found out," the girl told police.
The age of sexual consent in Britain is 16.
Portia Ragnauth of the Crown Prosecution Service told reporters that "nothing detracts from the fact that this was an abduction," she said.
Bishop Bell school, where the two met, said staff were "deeply shocked" by the affair but insisted: "Our safeguarding procedures are robust."