Forrest, 30, pleaded guilty to five charges of sexual activity with a child, a day after he was convicted of child abduction.
The teacher, originally from Aberdeenshire, admitted the new offences as he appeared at Lewes Crown Court to be sentenced for the abduction charge.
Forrest, who took the pupil to France last year when he was about to be exposed, was not originally charged with sex offences for legal reasons linked to his extradition.
Sussex Police last night revealed they are now investigating whether Forrest, who did not give evidence during his trial, contacted the teenager in a bid to alter how she gave evidence in court.
Judge Michael Lawson QC has questioned the evidence given by the teenager in court, as it varied from her police interviews and fitted with Forrest’s defence that he took her to France to prevent her from harming herself when she claimed she might have felt suicidal.
The judge told Forrest he had ignored the cardinal rule of teaching by entering into a relationship with a pupil. The court also heard he misled colleagues when he claimed rumours about the relationship were the teenager’s lies.
Children’s charities last night welcomed the teacher’s sentence, saying it highlighted society’s “disapproval” of such a betrayal of trust. Forrest had an intimate four-month relationship with the underage pupil.
The judge said Forrest’s behaviour had damaged many people. “It was her first intimate relationship,” said Judge Lawson.
He told Forrest he encouraged the girl’s infatuation and text messages showed he drove the sexual relationship forward. By taking her to France, he subjected the girl’s family to “appalling distress”.
Judge Lawson told Forrest: “Your behaviour over this period had been motivated by self- interest and has hurt and damaged many people – her family, your family, staff and pupils at the school, and respect for teachers everywhere.
“It has damaged you, too, but that was something you were prepared to risk. You now have to pay that price.”
He banned Forrest from working with children for life and ordered him to sign the sex offenders’ register.
The trial heard the pupil had just turned 15 when Forrest started a sexual relationship with her. The couple had sex in the teacher’s car, in hotels and at his home before Forrest booked them on a cross-Channel ferry from Dover to Calais on 20 September last year. They spent seven days in France before being spotted in Bordeaux.
The court heard statements from the mother of the teenager, saying she felt like “the worst mother in the world”.
The court heard Forrest had been in an unhappy marriage.
The jury took less than two hours to convict him of child abduction on Wednesday. After being found guilty, Forrest turned to the girl and said: “I love you,” as the verdict was about to be announced.
The girl, who was not in court yesterday for the sentencing, sobbed and said, “I am sorry” as he was led back to the cells.
Forrest’s mother, Julie, along with his sister Carrie Hanspaul, sobbed as the judge told a guard to take the defendant down.
David Tucker, head of policy at the NSPCC, said: “Jeremy Forrest’s relationship with this pupil was wrong.”