Marija Radojevic and her husband believed the trusted school was a cocoon where their son was safe.
Years later they discovered he had been sexually abused by one of his teachers.
"We believed that the school was a cocoon like our home," a tearful Ms Radojevic told the child abuse royal commission.
"This I believe is true entrapment."
Their son had a high profile in the Christian school he loved and trusted, but became withdrawn, surly and depressed in years 11 and 12.
It was only after he had finished school that he revealed he had slept with a male teacher, but told his mum he would deny it if she went to the police.
After many years of self-destructive behaviour, he admitted he had been abused but did not name the teacher.
"We are now left to deal with not only our endless feelings of loss and trauma, but also with the deep guilt about our naive ignorance," Ms Radojevic told the Sydney hearing on Wednesday.
She said the extent of the entrapment only became clearer after her son died in 2014.
Abuse survivor CAA said the early stages of sexual abuse do not feel like grooming and entrapment, primarily because as a young person he lacked the vocabulary to describe what was happening in those terms.
"This was part of the success of the grooming that I experienced. It was bound up in the fact that I wasn't able to say that it was grooming," CAA said in a statement read by counsel assisting the commission David Lloyd.
The perpetrator would say it was not abuse, but a form of love in their "special" relationship.
"There was also a sense in which the trust and respect I had for the perpetrator naturally engendered the idea that talking about the abuse would constitute a kind of betrayal both of the perpetrator himself and also of the relationship we had," CAA said.
Queen's University Belfast Professor Anne-Marie McAlinden said the early stages of grooming look like befriending and young people, as in CAA's case, might not have the language to name that.
The inquiry heard that rather than the common misconception, "monsters do not get children, nice men do".
Child Wise CEO Katherine Levi said when one school pieced together a popular and "cool" teacher's grooming behaviour, some of the now adult victims said they did not want him to lose his job because he was a good teacher and a nice person.
"It proves the point of the affection that people can have and the stereotype of it being someone that they won't like and who is not trustworthy and who is not popular."