High levels of asbestos have been found at Cwmcarn High School, a union official has said.
The school was closed until further notice on the afternoon of Friday October 12 while investigations are carried out.
Rex Phillips, of NASUWT Cymru, told BBC Wales he attended a meeting at the school on Monday. He said the problem was found when a company visited the school to carry out a survey on a boiler room.
He said the asbestos was found to be in airborne particles, with two-thirds of the school “inoperable”.
He said: “They have taken the action to close the building because of that and they have virtually got a sealed building at the school.
“But our primary concern this morning was for the health and safety of the pupils and staff.”
School cleaners had asked questions and been advised to register their possible exposure to asbestos as an industrial injury.
On Monday, Caerphilly County Borough Council said it was working with the Public Health service to analyse the findings of a structural report received by the authority on Friday afternoon.
Officials are due to reveal their views on the report today (Tuesday).
The council has also said it is currently working with the school’s senior leadership team and governors to explore alternative arrangements for pupils and staff. The council has admitted it is unlikely to be resolved this week and added that every effort was being made to accommodate pupils in Years 11, 12 and 13 as a priority.