Despite the illicit relationship, the man continued his teaching career for more than a decade before the allegations of abuse surfaced, and he was deregistered in 2019.

The man, 42, who cannot be identified to protect his victim and children, pleaded guilty to indecent treatment and maintaining a relationship with a child in the Brisbane District Court on Friday.

The teacher had been hired at the school in 2008 when the pair met.

The vulnerable teen had been fighting with family and was being bullied at school when she turned to the teacher for help.

The sexual relationship intensified over five months as the teacher kissed and fondled the teen before eventually asking her for oral sex, laughing when she vomited.

He also had the teen trim his pubic hair, but insisted there was no sexual penetration until she turned 16, the court was told.

As the teen's family relationship crumbled, the teacher became her guardian and gained legal control.

He also coached the girl to say he was just a "family friend" if questioned by outsiders.

Despite becoming engaged in December 2008, the teacher crafted an affidavit for the girl to sign denying any sexual relationship.

The teacher and student later married, having three children before the relationship ended, and revelations of sexual abuse surfaced during an acrimonious family court battle.

The court found the teacher had shown no remorse and took advantage of his position of trust throughout the relationship.

"She was vulnerable. She was in a metaphorical sense lost. She was looking for friendship ad no doubt affection," the sentencing judge said.

"You would communicate with her, and she gained some solace from her predicament through that communication."

Although accepting the relationship was consensual, he said there was no doubt the teacher had been a corrupting influence.

"There was a degree of opportunism in your conduct - you took the opportunity to become sexually involved with this woman, notwithstanding her young age.

"She was corrupted by you. You were the adult, and your conduct can only have acted to corrupt her."

The judge said the was no doubt the sexual relationship had deeply affected the teenager.

"It would be naive of me to think there has not been some impact, even though it was consensual," he said.

"It doesn't take away the fact of impact from the offending conduct, particularly given this woman's vulnerability at the time."

The man was sentenced to three-and-a-half-years in jail, suspended after 10 months.

Australian Associated Press