Nearly half the students from Beartooth Elementary called in sick Friday as health officials continue to work to find out what's making them ill.
In all, about 230 students were absent Friday, up from the 100 who called in or went home sick on Thursday. The school has about 460 students.
RiverStone Health is investigating the cause of the illness. Symptoms include nausea, vomiting and diarrhea.
The key to finding out what infected students are stool samples from those who got sick. When officials finish collecting samples, they'll send them to Helena for testing, which will take a couple of days, said Barbara Schneeman, director of communication and advocacy for RiverStone.
Given that it was Friday, district leaders and RiverStone Health officials don't believe all 230 kids were sick. They think some were kept home either for caution or convenience.
"We understand that completely," said Keith Beeman, Billings School District 2 superintendent.
Stephanie Landers, who has two children at the school, kept both home even though only her daughter was sick.
It was mostly a precaution for her son, she said. As of Friday evening, he still was feeling fine.
"I don't know what to make of it," Landers said.
Many parents had speculated that it was some type of food poisoning, she said.
But Landers doesn't think so. Both of her children pack cold lunches and did so all week.
RiverStone Health officials inspected the school kitchen Thursday and found nothing, Beeman said.
SD2 contracts its food service with Sodexo, which prepares breakfast and lunch off-site and delivers them to individual schools.
District custodians cleaned Beartooth on Thursday with bleach and will do so again over the weekend, Beeman said.
"We will be ready to go Monday morning," he said.