STUDENTS at a leading Perth girls school have launched a campaign for the right to bring same-sex partners to their school formal.

A group of more than 40 past and present St Mary's Anglican Girls School students have confronted school authorities and started a Facebook campaign to argue for better gay rights.

But they say school bosses are refusing to back down and have told them that bringing a same-sex partner to the school ball is "inappropriate".

WA Equal Opportunity Commissioner Yvonne Henderson said the school could be breaching the Equal Opportunities Act by discriminating on the basis of sexual orientation.

Kia Groom, 24, who graduated from St Mary's in 2003 is leading the campaign. She said she formed the online group St Mary's Anglican Girls School Diversity this month.

She said "there are students at the school who don't feel comfortable" and the school policy was "damaging".

Other former students claimed the school chaplain, who is a member of the Facebook group and supported acceptance of gay students, was fired for being "too different" and "open-minded".

St Mary's declined to answer questions when contacted several times this week.

Ms Groom said gay rights had been raised many times at the school and each year students had elected representatives to approach the principal about bringing same-sex partners to the formal. And each year they were denied. Students were now determined to change the policy ahead of the next formal early next year.

"To me that is just unacceptable and it just shocked me ... there was no further explanation as to why," Ms Groom said.

"As a result, my school ball experience was fairly sub-par because I didn't get to spend the night with who I wanted to ... the whole thing was tarnished."

Ms Groom, who is bisexual, said coming to terms with her sexuality was made more difficult by the school. She said it tried to "nip lesbian behaviour in the bud".

Association of Independent Schools of WA executive director Valerie Gould said schools could make their own policies.

The Education Department said it supported healthy growth and development of students and ensured people were treated fairly in public schools.

But Ms Yvonne Henderson said though there were some exceptions for religious schools, anyone had the right to lodge a complaint if they felt they had been treated "less favourably". "Our stance is the Act and the Act makes it quite clear that it is unlawful," she said.

Gay and Lesbian Equality WA co-convenor Kitty Hawkins said other public and private schools had similar policies.

Some public school students were required to meet school heads to "prove they were gay" or in a same-sex relationship before being allowed to bring a same-sex partner.

"I understand that many single-sex schools wish to foster environments where they are able to mix with other genders, but this is still an inadequate reason (to exclude same-sex couples)," she said.

"Same-sex attraction and trans-genderism are not contagious and allowing one or two same-sex couples to attend a dance together will not insinuate that the entire year will then follow suit."

Ms Hawkins said same-sex couples and trans-gendered students were bullied and teased, which often led to mental illness, self-harm, substance abuse and even suicide.

"Schools public or private have an obligation towards their students to ensure that they are able to learn within an environment that is safe, respectful and accepting," she said.

"To bar same-sex couples from a dance sends a strong message. For a young person in such an environment, this can be devastating."