Scots-born Jeremy Forrest is serving a five-and-a-half year jail sentence for having a relationship with the girl, and for fleeing to France with her last year when she was 15 and he was 30.
The girl, now 16, who cannot be named for legal reasons but is known as Gemma, had claimed she "groomed" the teacher, originally from Inverurie in Aberdeenshire. After he was sentenced at Lewes Crown Court in East Sussex, she said she would wait for him and hoped they would marry. Forrest's wife Emily had already initiated divorce proceedings.
Now Forrest's 60-year-old father Jim has broken the news to him in prison that Gemma has found a new boyfriend, a sixth-form student her own age, according to reports at the weekend.
A family source was reported as saying: "Jeremy is naturally devastated. He's really upset, but at the same time he's a realist. He half expected this to happen while he was in prison, I think."
Gemma had been banned from seeing or communicating with Forrest until she was 18.
She phoned his parents Jim and Julie, 57, to give them the news and to apologise. They were said to have considered her "part of the family" and that they would support them if they did get married after their son is released.
A source was quoted as saying: "Gemma was in tears when she told Jeremy's mum she was dating again. She knows how supportive Julie and Jim have been and doesn't want to see either of them hurt."
Last week she had reportedly admitted to friends: "It's become too much, trying to have a relationship with someone I can't see or talk to.
"It was just too hard to carry on with the relationship. But I just want one last conversation with him to explain why I decided to move on in my life."
Before their flight to France, Gemma had fallen out with her mother and had left home. Now they have made peace and she is now living with her mother and three siblings in the south of England. Her mother said to friends: "I'm so, so glad my daughter is back where she belongs, and now I want her to go on and enjoy the things any girl her age would.
"I have met her new boyfriend quite a few times now. I like him a lot and hope they can now settle down and enjoy being in a relationship together."
According to the Sunday Mirror, Gemma's mother said: "I'm delighted there has been no contact between my daughter and Jeremy Forrest.
"I've longed for her to move on. I just want her to put it all in the past now. I just want to be there for her all the time now. I'm anxious to give her all the love and support she needs."
Forrest befriended Gemma when she was 14 and had turned to him with personal problems. She was 15 when he started having sex with her. He would pick the girl up and they would have sex in his car, in hotels and at his marital home. He sent her indecent images of himself.
During the court case it emerged Forrest continued seeing the girl despite being warned by staff at Bishop Bell Church of England School in Eastbourne. Police were not alerted until seven months after concerns about a relationship first emerged, but officers still did not question the teacher.
Forrest, of Petts Wood, Kent, booked a cross-Channel ferry from Dover to Calais the day after the police visited Gemma's home. He and Gemma spent seven days on the run in France.
In an impact statement, Gemma's mother said the teacher had "robbed her daughter of her childhood".