Parents were notified late Tuesday immediately after the San Ramon Valley school board voted to close it down.
The decision was made following the board's receipt of a report stating the 1950s-era gym, which is made of concrete, would not hold up well if a serious earthquake were to hit the area, district spokesman Terry Koehne said.
"Based on that information, we have decided to close that gymnasium with the ultimate goal of demolishing the gymnasium," Koehne said Wednesday.
"The safety of our students and staff members come first," he said.
Team practices and physical education classes will continue in two smaller gyms on campus.
A scheduled Wednesday evening boys home basketball game was played at Dougherty Valley High School in San Ramon. District staff members are scrambling to find locations for the team's remaining home games.
Koehne said that wrestling matches, girls volleyball and girls basketball games will be held in the school's new small gym, which opens this week.
Locker rooms are also being moved in the wake of the closure.
The seismic report was part of a series of tests being done on the large gym as the district planned to modernize the building.
The board voted Tuesday night to build a new gym after they realized that the cost to renovate the gym and build a new one was about the same price, around $10 million.
Funding for the large gym project comes from a 2002 voter-approved $260 million bond measure to update school facilities.
In an e-mail to San Ramon Valley High School students, parents and staff members, Principal Joseph Ianora said that other buildings such as the old small gym "are better able to withstand earthquakes because their wood-framed construction makes them lighter and more flexible."
No date has been set for demolition of the large gym. The school's older small gym was scheduled to be razed in March, but that has been postponed.
The school, the district and the Measure A Facilities Oversight Committee will work to figure out that deadline, Ianora wrote.
The new large gym would be placed in the spot of the old small gym. The site of the old large gym could create the potential for an expanded aquatic center and tennis facility.
This is not the first time a school district gym has been closed down for safety reasons. In 2003, the then one-year-old $2.8-million gymnasium at Monte Vista High School in Danville was closed after recurring water leaks soaked the gym floor.
Sophia Kazmi covers Dublin. Reach her at 925-847-2122 or firstname.lastname@example.org.