Angry Anglican school alumni are planning protests and signing petitions decrying their former principals' decisions to campaign for the preservation of powers that would let them discriminate against gay staff.

Amid the controversy, the heads of two schools that refused to join 34 other schools in signing a letter to MPs, Cranbrook and SCEGGS Darlinghurst, said on Friday said they welcomed everyone, regardless of race, religion or sexual orientation.

"I want to make it clear ... that I don't want SCEGGS to have any exemption from any Discrimination Act or the Fair Work Act based on our religion," SCEGGS Darlinghurst principal Jenny Allum said in a letter to parents and students.

The Anglican Synod last week sent all federal MPs a letter signed by the principals of 34 of its schools. It said while schools would not expel gay students or staff in practice, they wanted to preserve the right to employ people who "support the ethos of the school".

The letter prompted a backlash from alumni. Ex-students from Abbotsleigh and St Catherine's in Waverley are circulating petitions, while there is also disquiet among graduates of schools such as St Andrew's, Shore and Roseville College.

A petition begun by St Luke's graduate Max Loomes has attracted more than 2500 signatures from graduates of many of the schools involved in the letter, such as Barker College, Blue Mountains Grammar, Shore, Arden Anglican School and The Kings School.