An "extremely evil" and depraved schoolteacher who groomed and "sexually tortured" a schoolgirl has today been jailed for four-and-a-half years.

The South Island teacher gave his teenage pupil sex toys, made her wear leather handcuffs, collar and a leash, and demanded she call him "Master", a court heard.

They engaged in sexual acts over four months in what a judge called a "semi-formalised submissive relationship".

The male teacher, a married father who was jailed at Christchurch District Court, cannot be named for legal reasons.

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He was granted permanent name suppression, as was the South Island school, to protect the victim, whose mental health has deteriorated after the "degrading and demeaning" crimes, and left her suicidal.

The girl's parents gave powerful, harrowing victim impact statements in court this afternoon.

They told how they felt the school had let their daughter down.

Her mother spoke about the devastation caused by the teacher.

"We pray our paths will never cross another [name removed]," she said.

The parents told how they had done their utmost to give their daughter the best start in life, only for the teacher's depravity to "all but destroy her".

The father described how his daughter's mental health had deteriorated. She has gone from a healthy A student, to a damaged girl on medication that has left her "like a zombie with dead fish eyes".

He feels that part of his daughter's soul has been stolen by the teacher, who he described as "extremely evil and dangerous".

Crown prosecutor Claire Boshier described it as one of the more serious cases to reach court, with "immeasurable" harm caused.

There was a significant power imbalance, with the teacher abusing his position of authority and trust.

Boshier described the teacher's actions as reprehensible, depraved, and despicable.

There was a high level of premeditation and grooming, the court heard, with an email relationship going on for two years before things became sexualised.

The teacher had a self-entitled attitude, Boshier said, and any remorse that might be present is only for his own situation.

The Crown supported applications for suppression, with Boshier saying that if his name was published, the victim's identification would be "almost inevitable" and result in catastrophic effects for the "extremely fragile young woman".

Judge Tom Gilbert agreed. He imposed permanent suppression orders prohibiting publication of the name of the offender, school, and victim.

The teacher pleaded guilty to two charges of sexual connection with a young person under 16 between January 5 and May 17 last year, and a charge of exposing a young person to indecent material.

The court heard how the teacher and child started communicating via school email before changing to personal email addresses, and messaging on Google Hangouts.

There were 7982 emails or messages, including nude photographs, between December 17 2016 and May 17 last year.

"The email content between the defendant and complainant was extremely sexual, with graphic details and descriptions of the sexual acts they had done and wanted to do with each other," the summary of facts states.

"It is clear through these communications that the defendant is [in] control of the relationship, he had described numerous degrading and humiliating acts he wants to do to the victim."

The teacher called her "Pet" and she called him "Master", the court heard.

Judge Gilbert said the man persistently groomed the girl.

He was in a position of trust, not just as an adult but as a teacher at her school, the judge said.

Judge Gilbert noted the "extraordinary impact" the offending has had on the girl and her family.

"It's fair to say … you have defiled this young lady and crushed both her and her family," the judge said.

Judge Gilbert jailed the man for four-and-a-half years but decided against imposing a minimum period of non-parole, saying it would be up to the Parole Board when he would be released.