The Australian Christian Lobby has condemned a move by a Sydney high school to allow male students to wear female uniforms, and to let students freely express what gender they identify with.

Students at Newtown Performing Arts High School have been told anyone can wear a skirt to school, and refer to themselves as either male or female.

They have also been told that anyone identifying as a girl can use the female toilets.

But while parents have welcomed the change to the school's uniform rules, the Australian Christian Lobby claims it is a step too far and that it effectively encourages children to make radical choices with their lives

The school, in Sydney's inner-west, prides itself on its progressive values.

In recent years the school has approved special requests, allowing students transitioning from one gender to the other to wear the uniform of their choosing, and to use the alternative toilet.

The difference now is that these students do not have to seek formal permission.

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Maya Saric is a parent with two children at the school, enrolled in grades eight and 10.

"We've been told, through the children, that they're allowed to choose from either of the gender uniforms," Ms Saric told AM.

"And they've been told that the school has dropped gender-typing. The child can decide for themselves how they perceive their gender."

Ms Saric said she had no objections to the change in uniform policy.

"If children are really struggling with that issue, then they shouldn't be... forced into pigeonholes by the school," she said.

"They're facing enough turmoil and change and crisis of their own. It's not really relevant what colour trousers they're wearing and whether they're wearing a skirt or shorts."

Another parent, Tony Ryan, has also welcomed the change.

"Change to the uniform policy means that kids who feel that they don't fit into a particular category feel safe," Mr Ryan said.

'Radical gender theory' entering schools: Christian Lobby

But not everyone agrees with the school's decision.

Students at Newtown Performing Arts High School can now choose to wear either the male or female uniform without seeking formal permission. ( Supplied: NSW Government )

Australian Christian Lobby (ACL) managing director Lyle Shelton told AM he was worried about "rainbow politics" entering schools.

"This whole questioning of what it means to be male or female and the idea that gender doesn't matter anymore, I think is radical gender theory, and it's finding its way into our schools," Mr Shelton said.

"I think people are wondering if this is where rainbow ideology and rainbow politics is taking us.

"Maybe we better take a second look at the whole agenda that is behind things like the same-sex marriage debate, etcetera."

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Mr Shelton said he was also worried boys wearing skirts could be bullied.

"I think the thing to remember is that no-one should be bullied at school," he said.

"We have very good anti-bullying programs. No-one should be bullied for any reason at all. But these gender theory ideas go way beyond anti-bullying to almost proselytising and trying to encourage and instruct young people in radical practices.

"A lot of parents are starting to ask questions about where on earth this rainbow ideology is taking us."

Newtown Performing Arts High School is yet to issue an official statement on the uniform policy change.