More than 15,000 primary school students have repeated over the past four years, with almost 7000 redoing kindergarten.

The second highest year level for students being held back is year 10, with more than 1130 students repeating last year.

In nine of the 11 school years from kindergarten to year 10, more boys repeated than girls in 2013.

Principals say they strongly advise parents against repeating because research overwhelmingly suggests students who are struggling academically are better off advancing.

Holding students back has also been shown to contribute to poor mental health, low self-esteem and feelings of shame.

Dr Helen McGrath, a leading Australian education and psychology researcher, said of repeating school years, there are few educational issues where the evidence is so damning, comparing it to “playing Russian roulette”.

“You need to be fairly sure that you’re prepared for the possibility that it may in fact set the child back,” she said.

There are also huge costs involved, with the OECD putting the annual cost of grade repetition in Australia at about $50,000 per repeater, taking into account the expense of an additional year of schooling as well as the delayed entry into the workforce.

In a report released last year, the OECD found about 7.5 per cent of 15-year-old Australian students had repeated at least once, which was less than the OECD average of 12 per cent.

Dr McGrath said she was glad Australian educators were starting to pay attention to the damaging effects.

New figures show the number of public school students repeating has been declining steadily, with the number of kindergarten repeaters dropping from 1,908 in 2010 to 1,485 last year, according to data from the NSW Education Department.

The president of the NSW Primary Principals’ Association, Geoff Scott, says students commonly repeat because of poor social skills or maturity levels rather than academic outcomes.

Australian kids start school younger than almost any other developed country in the world, up to two years ahead of students in top-performing countries such as Finland and Korea.

Mr Scott says it is always the priority of principals to progress students, unless there were extenuating circumstances such as missing considerable amounts of class time due to a lengthy holiday or illness.

“Going back some years, repeating was often the first port of call,” he said. “But now that we have more information about how children learn, the realisation from both parents and schools is that repeating isn’t going to make any difference, it’s only going to make them a year older.”

He said, while students might experience an academic bounce after repeating, the gains were minimal and short-lived.

“The reality is that the problem that’s caused that issue in the first place is still there and hasn’t been addressed by just repeating,” he said. We really need to intervene and tackle the issue, whether that be a learning difficulty, a socialisation problem or emotional issues.”

In high school, far more students repeat year 10 than year 7, 8 and 9 combined, with almost 5000 NSW students repeating year 10 between 2010 and 2013.

And the number of year 10 students repeating has increased over the past four years.

In 2013, 1139 year 10 students repeated, up from 1025 four years earlier.

The president of the NSW Secondary Principals' Council, Lila Mularczyk, said the spike was “not a phenomena that I have experienced or heard of”.

The NSW government raised the school leaving age in 2010 so that any student under 17 who wishes to leave school has to either enrol in TAFE, undertake an apprenticeship or be employed full-time.

“I really can’t see why there would be an increase in repeating at that point,” Ms Mularczyk said. “Year 11 is more flexible than year 10, so students can choose subjects that best suit their needs and interests.”

With a July birthday, Keelin O'Reilly was always going to be among the youngest or oldest kindergarten students in the state.

“She was in day care from 10 months old, so she had all the social skills and she was reading quite well for her age,” her mother Sarah O'Reilly remembers. “She wasn’t learning anything new and another year of not being pushed was a waste of a year in my opinion.”

So Keelin arrived for her first day at Eastwood Heights Public School as an eager four year old.

She was coping well but towards the end of the year her family moved to Victoria, where the starting age cut-off is April rather than July.

“So we came down here and she was 18 months younger than the oldest in the class,” Ms O'Reilly says of her now seven-year-old daughter.

She saw the way being younger than your peers was affecting her other daughter Morgan, who is six years older than Keelin, and also has a July birthday.

“There are physical changes that her friends have already undertaken,” she says. “And, as a mum, my biggest concern is that her friends are allowed to do things that I’m just not ready to let her do yet.”

With that in mind, she decided Keelin would repeat kindergarten.

“We presented it to her that the teacher had asked her to stay back to help with the new kids coming in and she’s such a little helpful Harry so she was really thrilled with that,” she says.

“She does know that when we go back to NSW she’s not going to be in the same year as her friends and that concerns her a little bit.”

smh.com.au