DaVinci High School in Buffalo was evacuated just after 9:30 a.m. Friday after an irritant in the air affected more than two dozen students and staff.
The affected were moved to the D'Youville College Center directly across Porter Avenue from the school. After a medical evaluation, everyone was cleared to resume their normal activities.
There were reports of a student releasing pepper spray into the air in or near a second-floor bathroom, but the exact nature of the irritant has not been confirmed.
Buffalo Schools Associate Superintendent Sabatino Cimato says everyone is safe and the cause is under investigation.
"There are different assumptions and students are saying different things, but we do not know for sure that it's pepper spray," Cimato said.
"We do not have the facts of exactly what happened, so we have to proceed with caution."
Students and staff finished out the day and were dismissed from the D'Youville College Center. A statement from the district Friday afternoon commended all involved for their "extraordinary response" to the situation. The district says authorities are speaking with a person of interest and checking video footage.
The school has been declared safe and classes will resume on Monday.