Police have charged former Preshil teacher John McMillan, who taught at the Kew school until 1996, following a long investigation.

The school has already reached a settlement with a former student who claims he was sexually assaulted.

A police spokeswoman confirmed that a 72-year-old Queensland man had been charged with offensive behaviour, an indecent act with a child under 16 and indecent assault on a male person in relation to historical sex assaults that took place in Kew and Gippsland between 1975 and 1992.

Principal Marilyn Smith wrote to parents on Tuesday and said the school had supported the police investigation.

"We would like to acknowledge the courage of our school community members who have assisted police in this matter and invite anyone wishing to contact the school to do so," she said.

Mr McMillan is due to appear in the Melbourne Magistrate's Court on February 19.

The Age revealed in April that Preshil, which is Australia's oldest progressive school, had been rocked by sexual abuse allegations.

In July 2013 a former student revealed on a Preshil alumni Facebook page that they had been sexually abused at the school. The post triggered a flood of further allegations by victims, former teachers and students.

Another alleged perpetrator who worked at the school is believed to have died.

A group of former teachers and alumni, including Georgia Butters Cain, is calling on the school to publicly apologise.

"Courage is Preshil's motto and the current administration has displayed none," Ms Butters Cain said. "However, for the victims, the recent charges by police, I hope, may be a step towards some recognition and a chance to perhaps temper what could be a daily trauma."

Preshil was the creation of founder Greta Lyttle, and her niece Margaret Lyttle, who was principal for 50 years, until 1994.

Its alumni include Olympic gold medallist Lauren Burns, comedian Gina Riley and world-renowned philosopher Peter Singer.