The odor of pepper spray permeated the middle school building at the private K-12 school on White Oak Road, forcing the evacuation of about 80 students and staff members, authorities said.
Students were taken to WakeMed after complaining of nausea, burning eyes and sore throats, authorities said.
"Some people went in and came out and started coughing and coming out. People kept yelling, 'Don't go inside,'" said 10-year-old Jillian Gieman, one of the students taken to the hospital.
Days after a chemical fire forced the evacuation of thousands of Apex residents, emergency responders turned out in force when a chemical odor was reported at the school. Parents and school officials said they appreciated the response.
"We may find in the end it was a pretty trivial matter. But in light of issues we've had recently in the community, it was important that some sort of response was at hand," parent Todd Gieman said.
Students returned to class in the afternoon after a hazmat team deemed the building safe.
"Everyone in charge assured us that there was no danger to all the students on campus," Upper School Principal Pat Fitzpatrick said.
School officials are trying to determine who brought the pepper spray to school and whether the discharge was accidental or intentional.