A former teacher has been sentenced to two-and-a-half years in jail for having a sexual relationship with one of her students for almost a year, when the child was initially 15.

The teacher, who was 25 when the abuse began, had pleaded guilty early to two counts of repeated sexual offending against the girl, including sexual penetration.

Judge Annette Schoombie accepted the relationship and the sexual activity was initiated and approved by the child but it was not legally consensual.

The abuse lasted from July 2015 until March 2016, when another teacher filed a complaint and the offender was arrested.

During sentencing, Judge Schoombie told the Western Australian District Court that the teacher's crimes were particularly serious because she had been in a position of authority and trust at the time.

"You did commit some very serious offending … particularly because you were in a position of authority and because you engaged in sexual conduct with her when she was under the age of 16," she said.

Child sexual abuse squad detectives also found videos and 111 images of the offending on the woman's phone.

The judge accepted the woman had not groomed the child, felt genuine remorse, was a low chance of reoffending, had high prospects for rehabilitation and the relationship was not just for her sexual gratification, but she did not accept defence counsel's plea for a suspended sentence.

She explained if a teacher became sexually involved with a student, jail must be the consequence, to ensure parents could be confident their children were being properly cared for at school.

"The message needs to be sent to the community that this cannot be allowed to occur," she said.

The court was told the relationship started after the student went to see the teacher about her problems, and a friendship developed where the teacher came to regard the child as a peer.

Judge Schoombie said the teacher had "cognitive distortions" where she started seeing the child as an adult with which she could have a sexual relationship.

The sexual abuse occurred at several homes and at school.

She will be eligible for parole in 15 months.