A former teacher from one of Australia's most distinguished families of judges has avoided jail after being convicted of filming up the skirts of his students and possessing child abuse material.

Robert Emmett, son of NSW Court of Appeal judge Arthur Emmett and Federal Circuit Court judge Sylvia Emmett, pleaded guilty in May to the charges, which included filming the private parts of children.

The former St Andrew's Cathedral School maths teacher, who is also the grandson of former chief justice of the NSW Supreme Court Sir Laurence Street, filmed three 14-year-old female students.

Police also found more than 9,000 child abuse images on a portable hard drive at his home.

Today, Judge Ian McClintock sentenced Emmett to a two-year Intensive Correction Order (ICO) to be served in the community commencing September 17.

As part of the order he must do 32 hours of community service each month, continue to see his psychotherapist and undertake treatment programs.

The judge called the child abuse material the "disturbing and perverted exploitation of children".

"It's natural to feel sadness and empathy for the children and anger to those who exploited them," he said.

However, Judge McClintock noted medical advice that Emmett would receive better psychotherapy treatment out of prison.

The court heard Emmett was on a "positive rehabilitation trajectory," that he has undertaken some book-keeping duties and was trying to get work.

Emmett was arrested in 2013 for possessing the child abuse images, some involving pain, bestiality and humiliation.

Emmett walked around St Andrews filming some students on his camera phone and on one occasion pretended to tie his shoelace while crouching down to film up a girl's dress at Town Hall Station in Sydney's CBD

The NSW District Court has previously heard Emmett was remorseful and had no prior convictions.

St Andrew's Cathedral School assures community

In May, St Andrew's Cathedral School sent a letter to parents of prospective students advising them that since Emmett's "reprehensible behaviour" was discovered he "has never set foot in the school again" and the three girls involved were offered full counselling.

"Prior to discovery of his wrongdoing, Mr Emmett's record was exemplary," the email read.

"There was no reason to suspect he was engaging in the kind of conduct of which he has been found guilty."

It said the school invited the Office of the Children's Guardian (OCG) to conduct an audit of its policies and procedures.