But in a fiery letter to the Commission's executive director Stephen Elder, Greens education spokesman Nick McKim said the party's backing of the Gonski school funding reforms and its pledge to invest $4.8 billion over four years to support students with a disability would benefit Catholic students.

Principals and school leaders have contacted the Greens, saying that they will not pass on Mr Elder's letter to students and parents.

"Perhaps that is because they understand that your letter is motivated far more by your close association with the Liberal Party than it is by a desire to act in the best interests of students at Catholic school and their parents," Senator McKim said.

One Catholic principal said he refused to circulate the letter because it was not the role of a school to tell parents how to vote.

"I respect the adults in our school community to make up their own mind about who to vote for," he told The Age.