A mother’s claim that her son was told he could wear a dress to school, as aired in an anti-same sex marriage advertisement, is false, a Melbourne principal says.

Cella White, whose son goes to Frankston high school, appeared in the Coalition for Marriage’s television commercial saying “the school told my son he could wear a dress next year if he felt like it”.

But school principal John Albiston said the offer was never made.

“She said it happened in a science class so we spoke to her son’s science teachers. They said it never happened,” the principal told the Australian Associated Press.

“I think it’s very disappointing that she didn’t meet with me to explore her concerns. If we had investigated it with her ... she would have realised it wasn’t happening.”

Albiston said White had previously raised concerns about the Safe Schools program, aimed at educating students about sexual and gender diversity. “I guess I wasn’t shocked [by her claims in this advertisement] because I’ve heard her claims so many times on the media ... she’s very outspoken,” Albiston said.

In a statement the Coalition for Marriage stood by White’s account and said her son’s class “was told that they could wear the uniform of their choice”.

A voluntary postal vote on whether to legalise same-sex marriage will be held next month unless it is blocked by a looming high court challenge.