When Sebastian* started high school at one of NSW's top selective schools, he had no idea the inner-Sydney campus would make him physically sick.

Now 14, he started year 7 at Fort Street High School in Petersham in 2017, but his mother Nicole Melanson said he soon fell ill.

"He was sick for the entire winter," she said. "He'd come home from school at 3-something and go to bed."

Sebastian is changing schools because of mould at Fort Street selective school in Petersham.

She said the family told the school when Sebastian enrolled that he was sensitive to mould, but they were not made aware of the condition of the school's buildings.