Quite worrying. Based on my personal experience and online research, food in Singapore cannot be left in room temperature for more than 4 hours, due to our hot and humid temperature. To be safe, food needs to be put in the refrigerator as soon as possible if not consumed immediately.
Hope all affected students get well soon.
Source: 110 Nanyang Girls’ High students fall sick during boarding school programme
Read more at https://www.channelnewsasia.com/news/singapore/110-nanyang-girls-high-students-sick-boarding-school-programme-10097196