The heads of 34 Anglican Schools in NSW have written to federal MPs urging them to protect exemptions in the Sex Discrimination Act that allow them to sack gay teachers.

Key points: The letter follows the Government's introduction of a bill to stop discrimination against gay students

The letter follows the Government's introduction of a bill to stop discrimination against gay students The Government is still considering whether to end the exemptions for teachers

The Government is still considering whether to end the exemptions for teachers The letter argues faith-based schools should have a right to employ staff who support their ethos

The letter was signed by principals from some of Sydney's most prestigious private colleges, including Abbotsleigh, Shore and The Kings School.

It was also sent to Prime Minister Scott Morrison and Opposition Leader Bill Shorten.

It follows the Government's decision to introduce a new Religious Discrimination Act to protect gay students.

The decision, during campaigning for the Wentworth by-election, was sparked by a leaked review into religious protections in Australia by former Liberal MP Philip Ruddock.

It recommended the legislative provisions allowing religious schools to discriminate on the basis of sexual orientation, gender identity or relationship status should be made consistent around the country.

However, the letter dated October 25 said there was little evidence that gay students or staff had been dismissed by religious schools.

"By and large across faith-based schools, the issue at hand is the right to employ staff who support the ethos of the school," the letter read.

The letter also detailed how teachers were not just educators but also "powerful mentors" for young people.

"It is not appropriate, for example, for a teacher to undermine or denigrate the beliefs and teachings of an employing school," the letter read.

"This is a reasonable expectation not only for the employing school but also of many parents and families who have chosen the school for their children's education."

'The exemptions should remain'

The principals' letter requests that religious exemptions stay in place until religious freedoms are legally protected.

"The current exemptions, however clumsy, in the Sex Discrimination Act 1984 are really the only significant legal protections available to schools to maintain their ethos and values with regards to core issues of faith," the letter said.

"A more general positive right would be far better, but until such time as religious freedom is codified in legislation, the exemptions should remain."

The principal of Sydney's Shore Church of England Grammar School Tim Wright questioned the need for a new act this morning on ABC Radio Sydney.

"There hasn't been any gay student expelled from an Anglican school in my memory, and there hasn't been a gay staff member sacked from an Anglican School in my memory," he said.

"At interview the question we ask is 'are you able to support the Christian ethos of the school?'

"Many of our staff live with their boyfriend or girlfriend … and we accept that."

The Government wanted to draft laws to stop discrimination against gay students in the last sitting period as a matter of urgency, but Labor and the Coalition were unable to reach a deal and the matter was deferred until the next sitting of Parliament.

The Government is still considering whether to end the exemptions for teachers, saying that ending discrimination against students is its first priority.

Federal Education Minister Dan Tehan said there was a balance to be struck when considering the protection of religious freedoms.

"At the same time we've got to get the balance right when it comes to discrimination based on gender, based on sexual preference," he said.

"... the Attorney-General is looking at this incredibly closely and he wants to make sure what he's recommended to the Cabinet ... gets that balance right."