Schools need to be prepared.

Victorian schools have been put on standby to help students distressed by "hurtful campaign material" ahead of the postal vote on same-sex marriage.

In a surprise move, Education Minister James Merlino​ wrote to every state primary and secondary school on Friday, requesting that they prepare to connect upset students to welfare staff and school nurses.

He also directed them to the controversial Safe Schools Program, Bully Stoppers and headspace.

"Sadly, there have already been reports of children and LGBTI families being targeted in hurtful campaign materials regarding marriage equality," Mr Merlino said.

It comes as LGBTI agencies prepare for a sharp rise in calls for help sparked by the same-sex marriage postal survey.

Micah Scott, the chief executive of  LGBTI youth group Minus 18, said calls to the service had doubled in the past month.

It has referred 35 young people to mental health services in the past month, twice as many as it usually refers.

"Young people are needing more support and feeling really distressed by what has been distributed," he said.

This material includes posters distributed in the CBD which claimed that 92 per cent of children raised by gay parents were abused and featured the slogan "stop the fags".

Mr Scott said images of young people on the organisation's website had been used by anti-gay marriage campaigners.

"Imagine being a young person who has proudly used their photo to make other people feel empowered and that is flipped on its head in a sinister way," he said. 

He said he had also received reports of homophobia in schools that had been sparked by negative material used in the "no" campaign.

Prime Minister Malcolm Turnbull has hit out at "disrespectful, abusive" language used in the same-sex marriage debate and urged both sides to campaign respectfully. 

Victorian Association of State Secondary Principals president Judy Crowe said the minister's letter was a "useful reminder".

"Groups bring issues into schools and use any avenue to stir up trouble," she said.

"Principals see it for what it is.They are there to buffer the influence of those negative group on their student population," she said.

Fairfax Media revealed last week that Australia's Catholic Church is threatening to fire teachers, nurses and other employees who marry their same-sex partner if gay marriage becomes legal.

It is understood that some principals in Catholic school have been comforting distressed gay teachers.

Mr Merlino said the government was proud of the work taking place in schools to make them welcoming and inclusive.

"We will continue to work alongside school leaders and staff to ensure our schools are places of respect, where all families are supported and valued as part of the school community," he said.