About 200 students of CHIJ Secondary (Toa Payoh) came down with symptoms of food poisoning, including diarrhoea and abdominal pain, last week.
The source of the contamination has yet to be determined, said the Ministry of Health (MOH), which was notified by the school last Thursday, when students began showing symptoms.
Most of the students who became sick have returned to school and none was hospitalised, said the school's principal, Mrs Karen Tay, in response to queries from The Sunday Times .
"The school is monitoring the situation and working with the relevant government agencies to ascertain the cause of the symptoms.
"As a precautionary measure, the affected students' classrooms and school toilets have been cleaned and disinfected," said Mrs Tay.
In a letter circulated to parents on Friday, obtained by The Sunday Times, the school also said that food sources in the school were being monitored as a precaution.
Several parents contacted expressed support for the school.
"I appreciate what the school is doing... they put in a lot of effort for the students," said a parent, who asked not to be named.
Her daughter came down with diarrhoea in the early hours of Thursday and the symptoms continued the next day.
Investigations are ongoing to determine the source of the infection, said MOH.