Parents have even been banned from nativity shows at the school in Penicuik, Midlothian, but children went ahead with performances and lessons as normal.
Around 125 of the 400 children at Mauricewood primary – and several teachers – have been struck down in the suspected norovirus outbreak.
The decision to keep the school open has been branded as “madness” by parents.
The bug can cause diarrhoea and vomiting and while most cases pass quickly the virus can transfer easily between people.
On Monday, 90 pupils were off sick and by Tuesday, the day of the shows, the figure had risen to 114.
Despite losing several members of the cast, the plays – one for junior pupils and the other for P3 to P7 – went ahead.
Parents, however, were told to stay away from the performances of “Born in a Barn”.
Parent Lesley Anne Liddell complained on Facebook: “There’s 119 kids off with the virus, but we’re still to take our well kids to school.
“They should close school… they’re gonna spread it.
“Carol concert and pantomime [are] cancelled too.
“Yet send them to school? Madness.”
She continued: “I don’t want my kids to catch it, none of us do.
“There’s teachers off also. Some classes are down to 12 pupils, its being deep cleaned but its too late,the virus has started it’s a matter of time.”
Parent Carrie-Ann Gordon posted: “If the virus is that bad that all Xmas activities are cancelled, should they not just shut the school?”
Councillor Bob Constable, Leader of Midlothian Council, said: “We take the health of our pupils very seriously and the Council is working very closely with NHS Lothian to monitor the diarrhoea and vomiting outbreak at the school.
“We are following advice from NHS Lothian who are calling the outbreak a ‘mild’ virus.
“At this stage, there is no need for a full school closure, but we will continue to monitor the situation and follow advice from the health board.”
Councillor Lisa Beattie (SNP), Midlothian’s cabinet member for education, said NHS Lothian recommended that parents stay away from the nativity performances.
Dr Christine Evans, a consultant in public health at NHS Lothian, said: “We are currently investigating a possible outbreak of diarrhoea and vomiting, potentially caused by norovirus, at Mauricewood Primary School in Penicuik.”
She said most cases were reported on Tuesday but the NHS was continuing to monitor the school.
“We are in daily contact with the head teacher and recommended yesterday that all potential trips and events, which could involve members of the wider public, should be taken at a later date as they are likely to increase spread of the infection.
“We have also advised that anyone attending the school who is displaying symptoms should stay off and not return for 48 hours after their symptoms have stopped.”