UPDATE 2:45 p.m.
Classes will resume Thursday. School officials said they'll bring in food and water for the students and staff.
UPDATE 1:12 p.m.
Nothing was found in the air after testing by the Missouri Department of Natural Resources.
Cole County Emergency Management Director Bill Farr said the first call for aid came around 8:30 a.m. when the Cole County Ambulance service was called to respond to one student who had passed out at Mass.
"More calls came in after that so we gathered more resources," he said.
Farr said Ameren Missouri crews did testing inside and outside the building and could find no leaks or strange smells at the school.
"The only common denominator is that all the kids affected did drink water at the school," he said. "Other than that it's a real mystery. We've never had any problems at this building."
UPDATE:
Authorities deemed it safe for students at St. Joseph Cathedral School to be allowed to go home. During the noon hour, parents picked up students. Authorities said no other children had gotten sick. Testing as still taking place, including the water and it could take as long as 24 hours before results could come back.
EARLIER COVERAGE
What caused several children to become ill at St. Joseph Cathedral School Wednesday morning is under investigation.
Jefferson City Police Department Sgt. Pat McCuthchin said some children apparently passed out for an unknown reason.
"At least one child collapsed while they were in church service at St. Joseph Cathedral," he said. "Parents were notified and sought medical attention for those children. Some even came back after this because they were fine."
"Later in the morning more kids became light headed, dizzy, passed out and some even had a rash," McCutchin continued.
As of 11:30 a.m. around 12 students had been affected.
"The first three victims went from the school to the church and then had their symptoms appear,"McCutchin said. "The other kids that were affected were in the school when the symptoms hit them."
Members of the Cole County Health Department, Jefferson City police and fire departments as well as the Cole County Hazmat team were on hand.
The Missouri Department of Natural Resources was on hand to test water an air quality at the school.
The children were not quarantined, but they were held in place on the school and church properties to make sure testing could be completed.