No students or school staff were hurt.
The fatal accident happened in a construction zone at Villa Park High School in Orange County, according to the school district.
OCFA personnel and the Orange County Sheriff's Department responded to the school about 10:21 a.m. and found two adult patients.
One patient was transported to a local trauma center, according to the Orange County Fire Authority. The agency said the second person was pronounced dead at the scene.
Neither of the workers have been identified.
"There is no current danger to the Villa Park High School campus or surrounding community," said the school district in a statement.
Students said the construction that was going on inside the gym was a renovation project.
"They took out all the new bleachers, they re-did the floor, new air conditioners and everything. I know there are new bleachers that were delivered a couple of weeks ago that they've been working on," said student Landon Hunt.
Hunt said at first he didn't know what happened because the only announcement that was made was that the school was going into lockdown.
"I was pretty scared. I thought it was a shooting or something because I wasn't sure," he said.
ABC7's news helicopter was over the scene as a body was wheeled into the coroner's van, and other construction workers appearing to console each other after what happened.
No further details were immediately known.