OAKLAND -- A strong smell of natural gas forced the evacuation of 1,800 students and ended classes early Tuesday morning at Skyline High School in the Oakland hills.

It was the latest in a series of natural-gas-related problems that have plagued the school since a contractor ruptured a line Feb. 17, causing the Skyline Boulevard school to delay its reopening from the Presidents Day holiday.

The strong smell was detected about 9:15 a.m. in a former science classroom near the school administration building. District spokesman Troy Flint said all students were evacuated as a precaution while firefighters and school officials investigated.

Students were sent home about 10:30 a.m.

A PG&E spokeswoman said a main gas valve on the roof that fed Bunsen burners in the former classroom lab was turned on, releasing natural gas into the classroom. Utility spokeswoman Tamar Sarkisian said she did not know how the valve was opened. PG&E crews responded to the school but it was later determined the utility was not at fault, she said.

School maintenance workers shut off and sealed the valve so it won't happen again.

Staff writer Kristin J. Bender contributed to this report.