He was found guilty of indecent acts against seven students, all younger than 16, such as kissing or touching their legs and breasts under the pretext of searching them for cigarettes.
The offending escalated to sex with a 14-year-old girl.
Gillard, 45, was arrested in July 2012 after the girl's grandmother contacted the police on the advice of principal Peter Gall, and detectives tracked down the other victims.
Although he denied the sex acts, Gillard pleaded guilty to supplying methamphetamine and attempting to obtain a false passport to flee New Zealand before trial.
In sentencing Gillard, Judge Charles Blackie said the former professional rugby player had "grossly abused" the trust placed in him.
"The community puts a high degree of trust in school teachers.
"Every day when a parent sends a child to school, he or she does so in the knowledge that the child will be safe ...
"A high standard is expected of school teachers in their dealings with their pupils because school pupils are vulnerable."
One of the victims was a 15-year-old student propositioned by Gillard who offered to pay her money for sexual favours in 2006.
"She did make mention of her discomfort to school authorities, but it would seem that no action was taken or the matter was not taken up," said Judge Blackie.
Ben Taufua, chair of the school board, ordered an independent review to into Papatoetoe High School processes and the report identified a lack of documentation as a problem.
A copy released to the Weekend Herald by Mr Gall shows two girls complained separately to senior staff in 2006 about feeling uncomfortable about their teacher.
One told the deputy principal that she did not like how Gillard tried to spend time with her alone.
"At that point alarm bells went off so I started taking notes ... I could tell by her body language she wasn't lying or making anything up ... the look on her face was one of almost fear," the teacher told report author Carol Anderson.
The student was also interviewed by another senior staff member and both teachers say the complaint was passed on to the "boss" assuming it would be investigated. But there are no records of the complaint and Mr Gall, the regular principal, was on secondment at another Auckland school so he was not told about it.
The 15-year-old abruptly left the school and the complaint was not able to be followed up. A few months later, a second girl came forward to say she knew "disgusting things" about Gillard.
She told a senior staff member that the first girl who raised concerns left school because Gillard offered her money for sex - something he was later convicted of.
The staff member does not recall passing on the file note, which later formed part of the 2012 police investigation, and the second complaint did not reach Mr Gall. "We knew in 2006 that things were happening with Gillard and it took more victims and more and more years for this to finally come out. I have had sleepless nights over this because there have been other kids who have been affected for their life and I feel guilty because we didn't protect them," the deputy principal told Anderson. The report notes that no "formal complaints" were made to be investigated, but a lack of records on Gillard's file made it difficult to keep track of a pattern of behaviour and "join the dots".
The school has since implemented Ms Anderson's recommendation that all complaints of a potentially serious nature should be referred to the principal in the first instance, rather than other senior staff, to ensure someone has full knowledge of all the issues.
School chairman Ben Taufua said there had been a "communication breakdown" and believed he would have been told if Mr Gall had been present at the school.
"I'm really sorry about this. We've failed that student. As the chair, I take full responsibility. It stops with me because one of my jobs is to make sure our students are safe," said Mr Taufua.
"One of the hardest things I've ever had to do was get up at a public meeting of our school community, with Peter [Gall] by my side, and apologise."
Documentation and complaint processes have since been tightened.
Gillard had been suspended on full pay since his arrest in 2012 until his convictions this year.
But a Weekend Herald investigation has also discovered he was previously stood down by Papatoetoe High School three years earlier because of sex and drugs allegations.
Because of the lack of documentation on Gillard's employment file, the 2006 complaints were not discovered by the 2009 police inquiry.
Documents released under the Official Information Act show Gillard was suspended by the board of trustees in February 2009 after being arrested on indecent assault charges against two girls aged 11 and 15.
The board's memo to the Ministry of Education shows the criminal offences "if proven, would place students at the school at risk".
His bail conditions meant he could not be with girls under the age of 16 by himself, but his identity was kept secret.
"The charges relate to historical offences ... dating back 15 years.
"If name suppression is lifted, New Zealand police feel other students (past and present) may come forward with information," according to the briefing note.
His name was not published and the prosecution case fell apart at the trial in October 2009.
One of the complainants did not turn up to trial, so the charges stemming from her allegations were dropped, and the the jury found Gillard not guilty of indecent assault and supplying cannabis in relation to the second teenage girl.
Mr Gall gave a character reference for Gillard at the trial as the charges did not involve Papatoetoe students - a decision which was queried by the independent report.
"I'd written an affidavit to say he had been a good employee of the school and it was literally a two-minute appearance in court," he told the Weekend Herald.
"He was acquitted so we had to take him back and we had no mandate to investigate as it didn't involve our school.
"But it was always in the back of my mind. Hindsight is a wonderful thing but most of our staff were blown away by the charges. There were a lot of angry and upset people."
He rejected rumours, referred to in the report, that he was close to the Gillard family.
"It's absolute nonsense. I'm a member of the same golf club as his parents, but I've never played a round or had a beer with them.
"As soon as I found out anything [about the 2012 allegations], I took the strongest action I could take and told them to go to the police," said Mr Gall.
Simon Lance, Gillard's defence lawyer, confirmed he had lodged an appeal against the length of his client's sentence.
Call for teacher discipline to have wider scope
The Teachers Council did not investigate the behaviour of Damian Gillard following his acquittal on drugs and indecent act charges at the 2009 trial.
The council, to be replaced by a new teaching authority, refused an Official Information Act request for correspondence about the earlier charges on the grounds of privacy.
Head of the council Dr Peter Lind said there had been a mandatory report to the Complaints Assessment Committee (CAC) but Gillard was found not guilty and there was "no further evidence that suggested he should be subject to disciplinary action" so the matter was not referred to the Disciplinary Tribunal.
"Please note that the CAC should only inquire into matters that are not identical to those which are raised in a criminal charge and which have resulted in an acquittal."
Board of Trustees chairman Ben Taufua said his "hands were tied" from an employment perspective because of the Teachers Council decision.
"He was acquitted so the Teachers Council didn't do anything, so neither could we."
But Mr Taufua said the new disciplinary body, Educanz, should take a wider look at prosecutions against teachers even when they were acquitted.
There was a difference between being found not guilty of a criminal charge and acceptable behaviour for a teacher, he said. "We should be setting a very high threshold for teachers. The threshold should not be guilty or not guilty ... what about the bloody behaviour? This new body should take a wider look than criminal liability in these sorts of cases. It should be about the facts."
Education Minister Hekia Parata said Educanz would make changes that will further strengthen disciplinary proceedings and address the ongoing registration of any teacher facing charges. These include the development of a Code of Conduct and all serious allegations will be automatically sent to the Disciplinary Tribunal.
