Ryelands Primary School has been sealed off after asbestos was found during refurbishment work.
Contractors were told to leave the site in Torrisholme Road today, Thursday, after the substance was found.
The school is currently closed for the Easter holidays and it is not yet known whether it will re-open next week.
It is understood an electrical firm from Preston had been on the site carrying out wiring and lighting repair work.
Around 20 workmen were in the building when the discovery was made.
A Lancashire County Council spokesman said: “Asbestos was discovered at the school when maintenance work was being carried out.
“As soon as we became aware of the issue we took immediate steps to understand the situation. Experts are currently inspecting the premises and will inform us of any and all actions that are necessary.
“The school is currently closed for the Easter holidays and we will not allow the building to be used until we are sure it is safe.
“As soon as we know more information from the experts we will be in touch directly with parents and pupils.”