Canon-MacMillan Senior High School in Washington County was evacuation Wednesday due to a "chemical gas release" according to the district.
The evacuation was done "out of an abundance of caution" the district wrote on their website and on social media.
The gas was confined to the school swimming pool area.
Students were served lunch outside.
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Students and staff were permitted to return to the building just before 11am after the North Strabane Fire Department and Washington County Hazmat investigated and cleared the return.