The gas leak was reported at Colonial Acres Elementary School at 17115 Meekland Ave. at 8:10 a.m., Alameda County fire spokeswoman Aisha Knowles said.
Barbara DeBarger, director of elementary education for the San Lorenzo Unified School District, said school staff smelled gas in the main office area, notified district maintenance personnel and decided to evacuate all students from the campus.
Classes were scheduled to begin at 8 a.m., and students who were just arriving at school were escorted to the school's rear field. Some parents took their children back home, DeBarger said.
Firefighters walked through the campus, located the leak and attempted to stop the flow of gas, but it continued to seep, Knowles said.
PG&E crews were called to the scene and found that a valve on a 2-inch riser was leaking, spokeswoman Tamar Sarkissian said. Crews completed repairs around 9:45 a.m., she said.