Father Judge High School will reopen Thursday after the Archdiocese of Philadelphia confirmed no toxic substances are present in the building.
The school was closed for three days after about 150 people reported to area emergency rooms complaining of eye and skin irritation after attending a cheerleading event in the Northeast school's gymnasium Sunday.
The Health Department cleared the school to reopen Wednesday, but the archdiocese decided to wait from test results from the contractor it hired to evaluate the issue.
Those results came back all clear, though neither the city nor the archdiocese were able to determine the cause of the irritation.
Though school was originally set to be closed on Thursday in honor of the Feast of the Immaculate Conception, a holy day, but classes are being held at archdiocesan high schools to make up days that were lost as part of the teacher strike at the beginning of the year.