The school captain of one of Sydney's most prestigious private schools has used her end of year address to unleash her honest opinion of the institution which she said left her feeling "hurt and betrayed".

Sarah Haynes, the outgoing captain of Ravenswood School for Girls on Sydney's upper north shore, told the assembled crowd of staff and students that she felt "let down" by the school she loved.

She questioned the values of a school system which she stated was increasingly driven by financial concerns.

"I don't know how to run a school but it seems to me that today's schools are being run more and more like businesses where everything becomes financially motivated, where more value is placed on those who provide good publicity or financial benefits," she said.

The 18-year-old from Roseville said once she became school captain, senior staff would censor her speeches in line with the school's distinguished image.

"Everything I wrote had to be censored by those higher up than me," she said in her address. "I was never trusted to say the right thing."

She was instructed not to conclude an open day speech earlier this year in which she wanted to tell families the school was less than perfect.

"I sent this to those in charge of me and received a reply: 'Great speech but change the ending. No parent wants to hear that the school isn't perfect'," she said.

Ms Haynes wrote two versions of her address ahead of the speech day: one to show staff and the other she planned to give.

"I wrote two speeches today just so I would be able to say that Ravo isn't perfect," she said.

Families pay up to $28,000 a year to send their daughters to the Uniting Church school which Ms Haynes said projects an unrealistic image.

"If the school can't admit it isn't perfect how can they expect adolescent girls to realise perfection is unattainable," she said.

Ms Haynes said her speech was not a personal vendetta resulting from the circumstances surrounding her sister's departure from Ravenswood but she hoped the school would learn from its errors in the same way it expected from students.

"We learn from mistakes," she said. "The only dangerous thing about mistakes - which I think Ravo may have lost sight of this year - is not being able to recognise and admit to them."

Her speech received a standing ovation from the students and some parents although she described the reaction from staff as "reserved".

Ms Haynes, who hopes to study medicine, said her parents were a "little shocked" by her words but supported her decision to speak out.

"It was definitely taking a risk but I respect the girls in my community too much to be insincere so I took that risk," she said on Sunday.

Afterwards, many students said her message resonated with them, with a video of the speech posted on You Tube receiving almost  6000 views.

"The image (the school) tries to project isn't real and I think it can be unhealthy," she said.

"I fell for this image that the school presented to me. I thought the school was absolutely perfect. To be valued within the school a lot of people feel they have to present the same image of perfection that the school does. It's important to know that to be valued within a school you don't have to be a model student."

Public education advocate Jane Caro said some private schools carefully shape their image to help justify their fees.

"For those elite schools, it's all about brand, it's all about image," she said.

"When parents choose to send their children to those schools, what they are buying is prestige, brand, image and status. When someone calls that status into question, it raises the question of what exactly the school is offering for the money."

She said the school should be proud of Ms Haynes's integrity in addressing its shortcomings."It's managed to produce someone capable of critical thought who can apply her intelligence to what she has observed around her and had the courage to call it," she said.

Chairman of the school council Mark Webb denied that Ms Haynes' speeches had been censored by senior staff at the school which he said provided a "safe and respectful" learning environment.

"Girls have the right to express their individual opinion," he said.

He declined to comment on the reasons for her sister's departure from the school.

Mr Webb addressed the school straight after Ms Haynes's speech and said he did not recall how the crowd responded to her claims.

Association of Independent Schools of NSW have been contacted for comment.