MELBOURNE, Aug 30 - Cella White, whose son goes to Frankston High School, appears 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 said on Wednesday.

"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.

A voluntary postal vote on whether to legalise same-sex marriage will be held next month unless it's blocked by a looming High Court challenge.