POTENTIALLY deadly asbestos found at Lismore's Richmond River High School has forced the closure of 12 classrooms.
On Monday, two classrooms were closed after "imperfectly sealed" asbestos was discovered in window sills in the classrooms.
Samples were taken which tested positive for the mineral fibres, which can cause asbestosis, lung cancer and mesothelioma if inhaled.
Yesterday, Richmond River High School principal Dave Harvey sent home a letter with students to inform parents of the situation and posted information on the school's website.
"After yesterday's confirmation to you that classrooms 24 and 25 had to be made out of bounds after it was found that there was imperfectly sealed asbestos in the rooms' window sills, we have taken the precautionary measure of making a further ten classrooms out of bounds," Mr Harvey posted.
"Alternative learning spaces are being arranged for the students who would normally use those classrooms.
"Remedial work is being undertaken by qualified contractors working to WorkCover guidelines."
"Between four and six of them (classrooms) will need more substantial remedial work which is expected to take at least two weeks," he posted on the school's website.
"I will keep you advised of developments and appreciate your support in helping the school make the timetable changes or other changes that will be needed in the short term."
NAPLAN testing and HSC students will not be adversely affected.
"Years 7 and 9 are taking the NAPLAN tests to schedule," Mr Harvey posted.
"I can confirm that Year 12 HSC students' normal face-to-face teaching time will be maintained, and that we are doing everything we can to maintain normal teaching time for all other students."
When asked whether the asbestos posed a risk to students' health, a NSW Education Department spokesman referred The Northern Star to the NSW Health website.
"Asbestos fibres can pose a risk to health if airborne, as inhalation is the main way that asbestos enters the body," the website states.
"Small quantities of asbestos fibres are present in the air at all times, and are being breathed by everyone without any ill effects.
"Most people are exposed to very small amounts of asbestos as they go about their lives and do not develop asbestos-related health problems."