The 72-year-old man, who cannot be named for legal reasons, sexually assaulted three of his step-children, two nephews and nine young boys at a public school.

The former North-West man earlier pleaded guilty to 18 counts of indecent assault and two counts of maintaining a sexual relationship with a young person.

More than 20 onlookers filled the Supreme Court in Hobart to see his sentence handed down, with several more watching from Burnie.

Outside court, one of his victims described the punishment as ‘‘weak as piss’’.

‘‘It brings me some closure to know he’s locked up but I’m absolutely gutted that he could be out in three years – it’s extremely disappointing,’’ the man said.

Justice Shan Tennent said listening to the man’s step-children speak of their abuse was extremely harrowing.

The youngest girl was just 10 when he began molesting the children in the early 1970s, her brother only 12 and elder sister 13.

''Clearly the impact of your abuse is profound,'' Justice Tennent said.

Later, he twice assaulted his wife’s nephews inside their bedroom at night.

On each occasion he molested one boy, said ‘‘God Bless’’, and moved on to the next.

Justice Tennent said the paedophile assaulted each of student victims in strikingly similar ways, following a pattern of abuse that spanned many years.

''Your behaviour over several years constituted a grave breach of trust and had a terrible effect on many people,'' she said.

The Education Department responded to admissions the man nursed, kissed and cuddled his students, as well as taking photos of a group of naked boys, by moving him to another school in the 1980s.

He continued to work as an educator until the 1990s.

Allegations against him resurfaced last year as a result of a Royal Commission into Institutional Responses to Child Sexual Abuse, and police pressed charges.

Justice Tennent said while the man's guilty pleas had avoided a lengthy trial, it was not until late last year he admitted any criminal wrongdoing.

However, Justice Tennent acknowledged praise the man received for recent community work, citing one referee who said it was as if the man was trying to atone for his earlier behaviour.

The 72-year-old will be eligible for parole after serving half of his six-year head sentence, with his name to be placed on the sex offender’s register after his release from prison.

People Protecting Children president Allison Ritchie urged the Director of Public Prosecutions to appeal the sentence, arguing it was manifestly inadequate and did not reflect the severity of the crimes.