Brindabella Christian College has asked parents to vote no in the postal survey on same-sex marriage, warning any changes to the definition of marriage could impact on the school's freedom to teach its beliefs.
In a four-page statement released last week, principal Bruce Handley argued changes to the Marriage Act may impact on the school's ability to teach and model its views on marriage and family, writing that questions remained on protections for religious freedom.
"There simply is no certainty around what legislation may be proposed, a YES vote is basically signing a blank cheque to the Parliament to proceed with changes," he said.
"Our reservations in relation to the proposed redefinition of marriage itself and, regardless of what views may be held on that issue, the genuine and very valid concerns we hold about our ongoing ability to continue to teach and model a Christian view of marriage we are, respectfully, asking all those associated with the school community to participate and vote NO in the survey.
"We ask this in love and out of love."
Mr Handley also wrote that lesbian, gay, bisexual and transgender people within the school's community were not "any less loved by God", arguing the same-sex marriage debate was one of policy and politics rather than a pastoral conversation.
"As a Christian school, we will continue to speak into both at the same time, we seek what is best both for our society and for those within our particular school community," he said.
Parents and Friends of Lesbians and Gays Capital Region vice-president Ivan Hinton-Teoh said the same-sex marriage survey was not about challenging the religious definition of marriage but clarifying civil laws.
"At the moment our civil laws - the civil institution of marriage - discriminates against people based on their sexuality or gender identity," he said.
"Faith organisations clearly have a right to express their views on issues that they find significant, but the most significant issue for us ... is the welfare of young LGBTI people and those that are questioning their sexuality or gender identity."
Other Canberra schools and systems have waded into the same-sex marriage debate after voting forms were sent out last week.
The Islamic School of Canberra has urged parents to vote in the voluntary survey but has not persuaded them towards either side, emphasising everyone's humanity must be recognised and respected.
The Covenant Christian School board will meet Monday night to discuss its approach to the vote and will likely choose to communicate its stance to the school community.
Principal Martin Keast will endorse the Nashville Statement, written by the Council on Biblical Manhood and Womanhood, which affirms marriage as a lifelong commitment between a man and a woman and denounces premarital sex.
Last week, Catholic Archbishop of Canberra and Goulburn Christopher Prowse described marriage between men and women as "one of the greatest treasures of humanity", drawing comparisons between environmental protection and "the delicate ecology of human relationships".
"Why be so strong on protecting the natural beauties of Australia but less so on protecting human beauties, like traditional marriage?" he said in a note sent to the Catholic community, including Catholic school principals.
"Let us do all we can to protect traditional marriage from any legal re-definition."
Trinity Christian School, which came under fire in 2013 for urging parents to lobby against the ACT government's same-sex marriage bill, has not communicated with parents on the survey. Emmaus Christian School has not and will not write to parents.
An Education Directorate spokeswoman said public school staff could support the LGBTI community with rainbow banners, clothes and badges, but were not allowed to wear any "yes" or "no" branded material.
"No correspondence from the Director-General has gone to parents," she said.
"Some schools may engage directly with parents who need additional support, but this will not be in any way to persuade a vote either way."
Mr Hinton-Teoh, founder of just.equal, shared a message for LGBTI young people: "Please understand that this is a moment in time, that the larger world is on your side, that your life is going to be beautiful, that this moment will pass and we will achieve marriage equality and this moment will be a dim recollection in a very short time."