Current and formers students have taken to Facebook in support of marriage equality after it was reported the school had distributed the letter from the Australian Christian Lobby.

Nearly 300 people have liked a Trinity Christian School Students and Alumni for Marriage Equality page since Saturday.

The page was created by former Trinity student Michael Mazengarb, who graduated from the school in 2005.

“A lot of people contacted me – friends and former students expressing their disappointment [in the letter],” Mr Mazengarb said.

“A lot of people thought that it went beyond the role of a principal.

“The message it sent to parents and students was inappropriate.”

The letter was sent to parents at Trinity Christian School last week in an email from principal Andrew Clayton, who wrote that the issue was "emotive" and parents should "respond according to your conscience".

"Although this is not a usual practice for me as principal, I believe it is an appropriate response given the potential this bill has to impact the very nature of our school," he wrote.

Comment has been sought from Trinity Christian School.

The ACL letter said there had been no consultation with church leaders about the bill and it would be "naive to think" that any exemption for churches under the proposed legislation "will not be attacked in the future"

Mr Mazengarb said the Facebook page had received a lot of support in a short space of time.

“I’m glad the page has had such support because it shows students who are at the school now, or those who have left, and who have struggled with their sexuality that there is a lot of support for them in the Trinity community,” he said.