Mill Valley police said in an alert around 10 a.m. that the students had been evacuated from the school at 700 Miller Ave., and that residents in the surrounding area were asked to shelter in place.
"In the interest of safety, students will be released immediately, and classes will be canceled for the day while law enforcement sweeps and secures the campus," the school said in a statement.
A reunification center was set up at the Mill Valley Community Center at 180 Camino Alto in Mill Valley.
Parents can pick up their students at Mill Valley Community Center, unless they have arranged for them to transport themselves home.