Uniformed police distributed Church of Scientology propaganda at Whyalla High School, contrary to guidelines for religious education.
A team of police officers visiting the school showed classes a DVD on living a moral life and distributed a booklet entitled "Whyalla High School presents the way to happiness, a common-sense guide for better living", written by Scientology founder L. Ron Hubbard.
One outraged parent who did not wish to be named said she had explicitly told the school her children were not to be given religious instruction.
She said it was worrying members of the police force, in uniform, were endorsing the writings of a "cult leader" in their visit to the school.
Other parents believed the booklet had come from the Education Department, she said, because of its cover reference to Whyalla High School.
But the contents were the same as that written by Scientology founder L. Ron Hubbard and featured on a website linked to Scientology, she said.
Education Department district director David Craig confirmed the situation and said neighbouring schools would be informed to ensure it did not recur.
"While staff at the school checked the contents of the book, it was only after the seminar they realised it was written by L. Ron Hubbard."
He said the booklet did not contain religious information and the officers did not speak about the book during the session.
"The school will write to parents to inform them of its distribution," he said.
The sole indicator the book was related to Scientology, he said, was the author's name written in small print in the copyright section.
Police Assistant Commissioner Graeme Barton confirmed that two officers had visited the school to give lectures on bullying and that the claims of religious instruction would be investigated.