Up to 21 pupils of the Christian Brothers School in Naas, Co Kildare, have been hospitalised with a suspected viral infection.
Some 27 boys failed to turn up for school due to illness yesterday morning, the CBS principal, Mr Noel Merrick, told ireland.com.
By lunchtime, 40 pupils were ill and others presented throughout the day with 'flu-like symptoms, he added. The symptoms include a raised temperature and headache. Some pupils also suffered vomiting.
In total, 18 pupils have been admitted to Naas General Hospital and three are in Tallaght hospital. Extensive medical tests are being conducted to establish the nature and cause of the illness. At least one boy was tested for meningitis, but the tests proved negative.
Most of the pupils affected are in the sixth year classes at the CBS.
Mr Merrick said the school closed at lunch time today and will not re-open until Monday morning. Parents of the 800 students at the CBS will be circulated with information about the boys' illness.
A spokeswoman for the Eastern Regional Health Authority said the indications are that the students are suffering from a self-limiting viral infection of the upper respiratory tract.
She said the virus was infectious and urged others to be vigilant for symptoms including an elevated temperature. Anyone showing signs of the virus should contact a doctor, stay in bed and take paracetamol and plenty of fluids.
Dr. Margaret Fitzgerald, Specialist in Public Health Medicine with the ERHA said all the boys were comfortable.
The ERHA Department of Public Health and the local Area Health Board are working to determine the cause of the infection.