Jonah Soewandito is half the size of his classmates and should be in his last year of primary school. Instead, the 11-year-old has leapfrogged to year 12 and is doing his HSC.

The Scots College boy is the youngest student sitting the exams this year and one of the youngest to ever do the HSC in NSW.

Jonah, who should be in year 6, will sit 2 unit maths and chemistry and will finish his remaining HSC subjects, including three and four unit maths, physics and English, next year.

The Bellevue Hill college identified Jonah as being exceptionally gifted when he was just four, and he skipped kindergarten and started formal schooling in year 1. When he was in year 3, he was accelerated to year 5 as it became clear he was years ahead of his peers.

Last year, the college organised for Jonah to study chemistry through the prestigious US-based Johns Hopkins Centre for Talented Youth and the school has been in talks with universities in Sydney about options for Jonah once he finishes his HSC next year.

Some of his classmates may be six years older than him, but Jonah says he does not feel out of place learning alongside them.

While the rest of the state's year 12 students have finished school and are at home cramming for their exams, which start on Monday, Jonah is at school every day and fits in study around his other subjects.

He said he was studying hard for the maths and chemistry exams but was not feeling any stress. "It helps coming to school because it is a break from studying," Jonah said.

The school says Jonah's love of science was obvious early, because as a preschooler he could identify complex human anatomy.

"I like science because you are at the frontier of new discoveries," Jonah said.

"I would like to find cures for major illnesses, especially cancer, because it is one of the most insidious diseases."

Jonah's talents are not restricted to maths and science and he hopes to use his creative writing skills to publish a novel one day. He also plays percussion and enjoys soccer and basketball.

Chris Metcalfe, the school's dean of applied science, has been working with Jonah since he was in year 3, by which time he was already studying the equivalent of year 11 chemistry.

"Jonah is the most exceptional student I have ever taught," Mr  Metcalfe said.

Mr Metcalfe said the school worked hard to ensure it could meet Jonah's academic needs as well as his emotional needs.

"The one thing that worries us is how Jonah is doing emotionally but what we have found is that Jonah is more comfortable around his intellectual peers," Mr Metcalfe said.

"Jonah is a bit of a celebrity around the school and he used to be known as the smart kid but now the older students know him just as Jonah and they look out for him".