Someone called police around 9:30 p.m. to report a loud explosion and smoke coming from the parking lot of the high school, located at 201 Fair St., Lt. Tim Lyons said.
The person who called police saw a 1990s dark green or blue Toyota 4Runner with chrome rims drive out of the parking lot, Lyons said.
The Petaluma police and fire departments responded and found that someone had used chemicals to explode the bottle, Lyons said. A second, empty bottle was found nearby.
Lyons said there have been chemical explosions involving plastic bottles in the city before. There was no damage to school property from Saturday night's explosion, Lyons said.
Anyone who saw suspicious activity at the school Saturday night is asked to call Officer Pat Gerke at (707) 778-4372.