Asbestos was originally detected at Castle Hill High School six years ago but teachers say they were wrongly told the testing had come back negative.
"The most dangerous forms of asbestos, exist in that school," Castle Hill MP Ray Williams said.
"What worries me is not knowing about it – this is the first I've heard!" a father at the school said.
Williams has exposed 2016 testing which allegedly found asbestos in the school and he claims teachers at the school were told the tests were negative.
A professional standards investigation is now underway into these claims.
Since 2016, an entire cohort of students has gone right through from year 7 to 12.
Williams also claims staff have made ongoing complaints about "falling white powder" from "the ceilings, onto their desks, onto themselves".
The NSW education department issued a statement that it is "confident there are no current asbestos issues at the school".