The girl, who cannot be named for legal reasons, said Jeremy Forrest, 30, felt that forming an intimate relationship with her was a “risk he was willing to take”.
In a videoed police interview played to a jury, she said: “He did look up how many years he would be in prison, not for taking me to France but what had previously happened, and he told me that.”
The girl also revealed how they shared their first kiss in a classroom when she was 14, around the time he told her he loved her.
The kiss happened at Bishop Bell C of E School in Eastbourne, East Sussex, after the girl had turned up early in the morning to see him.
In the interview, the girl, now 16, said: “I do remember being in his room and it was just me and him. He said, ‘I really want to kiss you’ and I said, ‘I really want to’.
“It got to the point where there was only so much of a friendship you could have before it developed into something else.”
She went on: “It ended up being in his classroom.”
Lewes Crown Court heard that as their relationship became sexual, two months later after she had turned 15, Forrest voiced fears he could go to jail and lose his job over their affair.
The girl told how maths teacher Forrest researched how long he could face behind bars for having sex with a child.
She also claimed she had sex with him in the spare room of his marital home in Ringmer, in a Premier Inn hotel and in his car. Pictures were exchanged between them, including topless ones of her, she said.
Speaking about how Forrest behaved with her in the period after they first had sex, she said: “He was nice. He was normal. He was exactly how he’d been the whole way through.”
Asked about the possible consequences, the teenager said she was aware she was under the age of consent but the difference between them did not matter.
Forrest had told the girl his marriage to wife Emily was over and that he had stopped wearing his wedding ring. He also told the girl he wanted her to meet his parents.
And she spoke about events leading up to their disappearance in France and the call she received from her “calm”-sounding mother, who asked her to come home following a visit by the authorities.
Immediately suspecting that police and a child protection officer wanted to talk about her relationship with Forrest, she went to see him while he was doing a revision class.
The girl said she was going to leave that night but then decided to think things through.
The plan to go to France was “kind of my suggestion” and she said she encouraged Forrest to go with her.
She said they had spoken about running away together before, but only in a “jokey” sense.
Ferry tickets were booked for them under Forrest and his wife’s name before they headed to the Port of Dover on 20 September last year.
Forrest jotted down the phone numbers of relatives, including his mother and father, before tossing his mobile phone into the sea to avoid being traced, the girl added.
No passport checks were done on the girl at the port and after they boarded the Dover to Calais service, the atmosphere between the pair became more relaxed, she said.
The girl said they had sex in a hotel and they wrote journals which she thought could later be written as a book to “document everything”.
They then booked into a second hotel the following night, and over the coming days prepared CVs to apply for jobs.
Forrest visited a local newspaper website at an internet cafe and saw that their disappearance was the most-read story on it.
“That scared us,” the girl said.
During their time in France, the girl said she spent time in the hotel or looking around shops, while Forrest sought work and went busking.
The pair’s time in France came to an abrupt end when they were stopped by French police in the street. As they were held apart by officers, the pair mouthed “I love you” at one another, she said.
Forrest, of Petts Wood, Kent, denies child abduction.
The case was adjourned to tomorrow morning.