Garry Leslie Marsh, 72, repeatedly molested the student after taking him home and into his bedroom, the court heard during his trial.

The boy was about 12-years-old, and Marsh was his rugby coach.

After training Marsh would offer to drive the boy home.

The court heard Marsh would take the boy to his bedroom and take vaseline from the bedside table to touch the boy.

And Marsh would make the victim touch him, the jury heard.

One incident happened during a sleepover at Marsh's house.

It took the jury about five hours to return its unanimous verdicts.

Marsh was found guilty of 10 counts of indecent assault and one count of buggery, all against the same victim. The Sydney man was also acquitted of one count of indecent assault.

One of the jurors was crying as the verdicts were read out, and she was comforted by the other jurors.

Marsh had his head in his hands and shook his head at the verdicts.

Marsh was sacked from the college after the boy's mother told the headmaster about allegations of improper conduct, but the victim did not go into detail and he kept his silence.

In 2014, prompted by media coverage of the Royal Commission into Institutional Responses to Child Sexual Abuse, the victim reported the allegations to the police.

Prosecutors during the four-day trial in the ACT Supreme Court argued Marsh had a sexual interest in the boys in his care, and would grab the students on their bottoms or crotch on a daily basis.

Justice John Burns thanked the jury for a difficult task and sent them home.

He granted Marsh bail to await sentencing and to organise care for his mother, for who Marsh is the sole carer.

A custodial sentence was likely, the court heard.

The case will return to court in December.