The escapee, identified as Benedict Romero by the California Department of Corrections and Rehabilitation (CDCR), was located and apprehended around 1 p.m., approximately an hour and a half after the school was placed on lockdown.
Romero was found on an athletic field at San Gabriel High School, Alhambra police said. The lockdown was lifted after Romero's apprehension. No one was injured, police said.
Romero walked away from a residential home in the Alternative Custody Program (ACP) in El Monte on Tuesday, CDCR officials said in a news release.
The 39-year-old took off after tampering with his ankle monitor, and authorities noticed he was gone at around 9:35 a.m. CDCR officials said Romero had been participating in ACP since January.
San Gabriel city officials posted on Twitter earlier that the school was not directly related to the search, though they advised people to avoid the area.
ACP is a voluntary program developed for eligible offenders that allows them to serve up to the last 12 months of their sentence in the community in lieu of confinement in state prison.
Romero was received by CDCR in May 2016 with a two-year, eight-month sentence for vehicle theft. He was expected to parole in December.
Nearby Martha Baldwin Elementary School was also briefly placed on lockdown due to the search.