A sulfur-like smell drove students from Paulsboro High School this morning.
Students were moved outside, not sent home, for less than an hour, while the school was ventilated.
"They were evacuated because the smell on the outside was less than the smell in the building," explained district superintendent Frank Scambia.
They returned by about 9:30 to finish a full day of classes, he said.
Investigators were continuing to try to determine the source of the smell.
A police lieutenant, who compared the odor to rotten eggs, said the problem was sulfur dioxide released from a nearby oil refinery.
Nearby Paulsboro Refining Co. might be to blame, said a spokesman, reversing an earlier denial.
An "operational upset" occurred this morning, said Michel Gayda of PBF Energy, the Parsippany, N.J.-based company that owns the plant.
He declined to elaborate except to say, "There was no explosion, no fire, no injuries."
Notes would be sent home to parents and guardians explaining what happened, the superintendent said.
The PBF plant, which can process 12,000 barrels of lube oil a day, was acquired in December from Valero Energy, which racked up several million dollars in fines from New Jersey for various pollution infractions.
The EPA monitored air near the high school from August 2009 to February 2010, checking for toxic levels of acetaldehyde, benzene and nickel. A report released in March found no reason for concern, and monitoring was suspended.
"We look forward to working with all of the local constituents to ensure that the plant is run safely, responsibly and reliably," PBF CEO Thomas Nimbley stated at the time of the acquisition.
The plant brought PBF's East Coast refining capacity up to 375,000 barrels per day.