Police responded to several reports of a suspicious man dressed in camouflage and wearing a gas mask at Mountain View High School, located at 3535 Truman Ave., at about 8:20 a.m., police Sgt. Sean Thompson said.
Officers set up a perimeter around the school and nearby homes and searched for the suspect, Thompson said.
Investigators contacted a student in a class at the school, and he admitted to the prank, Thompson said.
The school was given the all-clear at about 8:55 a.m., the sergeant said.
The student, Christopher Egerton, of Mountain View, was arrested for causing a disturbance on school grounds, Thompson said.
The high school was not locked down during the search and there was no threat to students' safety, he said.
Thompson said a student had dressed similarly several months ago, but that police have not yet confirmed it was Egerton.