"I want to do advanced medical science at UNSW, but I need an ATAR of 96," Mr Hanna said.

"I'm doing my best but I don't know if my marks are good enough. I hope I can do better in the HSC."

Unlike most students considering a medical degree, however, Mr Hanna knows exactly what he's getting into.

Before he came to Australia as a humanitarian refugee in 2012 with his wife and three children, Mr Hanna worked as a general physician for 17 years in a village near the city of Mosul, which has since become an Islamic State stronghold.

He says he also worked with emergency services to get medicine to people in Iraq's mountain regions but had no enemies until 2010, when he and his family suddenly started receiving death threats.

"We went to Lebanon and spent two years there before we were accepted by Australia as refugees," Mr Hanna said.

He enrolled in an English language course two months after he arrived, and started year 11 two years later.

"I have to become a doctor again because in my previous life, I was only a doctor," he said. "I don't know how to do anything else."

Overseas-trained doctors can have their qualifications recognised by the Australian Medical Council but if they are unable to obtain proof of their degrees, as is the case for Mr Hanna, they need to repeat their studies.

This typically involves a six-year undergraduate degree followed by at least a year of postgraduate clinical training to become a medical practitioner, and at least three years of training to become a general practitioner, according to a spokeswoman for UNSW.

Mr Hanna says he is a very different student now than he was in the 1990s when he completed his bachelor of medicine, bachelor of surgery the first time around.

"I'm very nervous about my exams because I'm not doing as well as I use to," Mr Hanna said.

"All my life I was first in school but I'm older in age now and my memory is not like it was, and I have responsibilities."

Despite this, Mr Hanna is still near the top of his class in all his subjects, which include English as a second language, general maths, Arabic continuers and extension, and biology.

He is the oldest of 12 students over the age of 30 who will be sitting the HSC this year.

While a full profile of this year's HSC cohort is not yet available, last year's figures show 497 students sitting the exams were over the age of 20.

At Bankstown Senior College, which has one of the largest cohorts of mature aged students in NSW, 57 out of 71 year 12 students were over 20 last year.

Mr Hanna says having a mix of 17 and 18-year-olds and mature aged students in the classroom has been a great experience, especially when it comes to understanding his own daughters who are in year 11 and year 5, and his son, who is in year 8.

"It's like a family here, and I can understand young people and their different culture better," Mr Hanna said.

"And my own studies mean I can understand how the HSC and ATAR systems work, and I can discuss them with my children and help them take decisions about which subjects they choose and their futures."

He says one of his favourite things about studying for the HSC is competing with his 17-year-old daughter, who is doing many of the same science and maths subjects he did in year 11.

"We can discuss things and get help from each other, it's a good competition with my family," Mr Hanna said.

"My daughter is very smart, smarter than me. She's doing much better than me."

Mr Hanna's wife is also enrolled at Bankstown Senior College, and is in year 11 this year.

Mr Hanna says she wants to become a nurse and is also doing general maths, biology, Arabic continuers and English as a second language, but won't study with him.

"My wife doesn't like to study with me because she says I might be judgmental," he said. "She won't let me compete with her."

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