Students and faculty were told to exit Philadelphia Academy Charter High School on the 1700 block of Tomlinson Road just before 7:30 a.m. Wednesday.
Chopper 6 was overhead as students gathered out front of the school.
Philadelphia Gas Works was notified and students were allowed back inside about an hour later.
Investigators say there was a spill in the chemistry lab which caused the odor.
It was cleaned and cleared by a hazmat team.