A bout of diarrhoea and stomach pain struck 46 Punggol Primary School pupils and five teachers and forced the early closure of a three-day camp.
Some of the pupils also vomited.
In response to queries, the Ministry of Health and the Singapore Food Agency said investigations on the incidence of gastroenteritis among camp participants are ongoing.
Principal Hanafi Asmore told The Sunday Times yesterday that three pupils had first reported symptoms on Thursday night - the first day of the three-day camp.
"Of the affected pupils, 30 said that they were well enough to continue and 15 were sent home," he added.
One pupil was hospitalised for observation and discharged yesterday.
The school decided on Friday evening to end the camp in the interest of student well-being, said Mr Hanafi.
He added that the school is working with government agencies to determine the cause of the problem while parents of the affected pupils have been advised to monitor their children's condition.
The camp for about 200 pupils was held at Dairy Farm Outdoor Adventure Learning Centre run by the Ministry of Education.
Pupils were reportedly told to bring their own utensils, a lunchbox and a cup for meals, which had been catered for and laid out buffet-style.
A pupil said that after some of them fell sick, packets of food were bought from outside.
Around five busloads of pupils were taken back to the school on Friday night.
When The Sunday Times visited the school in Hougang, a large group of parents were waiting at its main gate to take their children home. Many said they were worried when told that some of the children had fallen ill.
Primary 5 pupil Janine Chia, who suffered from stomach pain, said she and some of her schoolmates felt better after resting in the dormitory for a few hours.
"But some of them who were more ill went home earlier," she added.
Her father, crane operator Jeffrey Chia, said the school had made the "tough but right" decision to abort the camp.
He will be taking Janine to the doctor for a check-up, he added.