In sentencing him, Judge Lawrence Hinton said the teacher's "very serious offending" was a breach of trust because he was related by marriage to the girl.
He took into account the teacher's remorse and his early guilty pleas in sentencing him to perform 200 hours of community work.
"One thing I add into the mix here is that to an extent, you may have suffered the loss of a career as a result of a conviction," Judge Hinton said.
But the teacher did not lose his career and is back in the classroom. That has upset his victim's mother, who says he is teaching girls the same age as her daughter was when she was abused.
The woman, who cannot be named to protect her daughter's identity, said the school had a moral obligation to protect its students and tell parents about the teacher's convictions.
"I have no reason to believe anything has changed with him. I have no reason to trust him with anyone else's daughters. I just wouldn't."
The woman said she approached the Teachers Council and the Ministry of Education last year when she found out the man was hired by the school. Her husband met the principal to discuss the teacher's convictions.
"Not one person will take responsibility for this," she said.
Her daughter had been badly affected by the abuse and suffered from depression.
"There is no end for the victim; it never fully ends."
The principal confirmed he and the board of trustees knew of the teacher's convictions when he was hired.
He said 21 years had passed since the man committed the crimes, he had not reoffended and was well respected by students and staff.
Asked if he had any concerns, he said: "Not an ounce for this man, not an ounce."
The principal said some parents knew of the teacher's convictions but they had not been "advertised".
"He's led an exemplary life for the last 21 years, fully recognising what he did was totally wrong. He's turned around and done everything right."
Teachers Council director Peter Lind said the teacher did not have a full teaching certificate.
Asked if the man could be alone with students, Dr Lind said he could not comment.
"There are tight conditions and there is regular monitoring." he said.
"There have been reports on a regular basis from the school that he is performing professionally and adhering to his conditions of practice."
The principal said there had been no complaints about the teacher.
"Nobody is sitting in every class lesson, but we have a thorough appraisal programme where I and the deputy principal visit classrooms."
Last week, it emerged that another teacher, with several aliases, taught at several schools despite three convictions for indecent assault and two of assault on his 14-year-old nephew.
