The principal of an elite Sydney private school has reminded parents that the fees they pay do not entitle them to behave aggressively towards teachers.

The head of St Andrew's Cathedral School, Dr John Collier, wrote in a recent newsletter that he was “very displeased at the current level of agitation from a minority of parents”.

He said the school has to deal with “too many parents who have verbally abused, physically threatened or shouted at a staff member”.

The school charges $18,200 for kindergarten students and $31,300 for students in years 11 and 12.

Dr Collier said that protecting his staff means telling them not to answer phone calls or emails from demanding parents in some cases.

He even threatened to ban them from entering the school.

“I am aware some parents — because they are paying fees — see the relationship with teachers as a master/servant relationship, such that they are entitled to make extravagant demands,” Dr Collier wrote.

“Even in a mercenary sense, this is hardly true.”

As parents, “we tend to react when we perceive our child is threatened”, he said, adding that this could “bring forth a reptilian kind of defensive response”.

He said that in one case, a middle school parent told him that 13 staff members who observed his daughter misbehaving were all lying.

“It is very hard to make progress with this level of unreality,” he wrote.

“Recently, a middle school parent said to me that as her daughter had done poorly in her test, her life was actually over.

“Actually, it wasn't.”

Dr Collier said only a small percentage of fees from parents go into teachers' salaries, but it is “hardly a sufficient commercial relationship” for parents to influence the craft of teaching.

“I take it this drift is part of a general decline in civility in society and needs to be called out,” he wrote.

Dr Collier said that in his 28 years as a principal, he observed “a considerable increase in parental anxiety” which “may reflect increasing anxiety in society generally”.

“As our children would say to us: ‘Chill!’”

Deputy president of the NSW Secondary Principals Council, Craig Petersen, said public schools have seen a rise in parents behaving aggressively and making unreasonable demands.

“There's a sense that you're a public school, you belong to the public and I'm a member of the public therefore I'm your boss — which of course is an overly simplistic way to view it,” he said.