More than 100 pupils at St Modan's High have been hit by a suspected outbreak of the norovirus.
The youngsters at the Stirlling school, which also draws pupils from the Denny area, have been suffering from sickness and diarrhoea.
With pupils now off for the weekend extensive deep cleaning is taking place.
An NHS Forth Valley spokesman said: "Many pupils chose to stay away from school but with a near-full complement of staff, lessons continued as normal.
"Any pupil who feels unwell should be kept away from school and those who have been ill should not return until 48 hours after symptoms disappear."
"Diarrhoea and vomiting is a common, self-limiting illness with many cases currently affecting the community at large; it was therefore felt appropriate that the school should remain open."