The extra staff was assigned to help students and teachers Monday at Twin Peaks Middle School in Poway.
On Saturday, a 12-year-old boy was taken into custody and sent to a hospital for evaluation and treatment after the seventh-grader sent a threatening email to a Twin Peaks administrator, according to the county Sheriff's Department.
His name wasn't released.
In the Friday night email, the boy claimed he would shoot one of his teachers and 23 unidentified classmates during second period on Monday, sheriff's officials said. He claimed to have 3,000 rounds of ammunition and access to a rifle and handguns, authorities said.
The Sheriff's Department worked with the school district and a computer crime task force to track down the source of the email, sheriff's officials said.
Investigators seized computers and weapons from the boy's home. Five rifles, three shotguns and three handguns were at the home but they were locked up and the boy couldn't get to them, sheriff's Capt. Bill Donahue said.
The boy's parents cooperated, he said.
Schools around the nation have been taking extra precautions to safeguard students since a gunman killed 20 elementary school students and six teachers in a Dec. 14 attack in Newton, Conn.
"Obviously, this situation is both saddening and disturbing," but procedures in place to deal with threats worked, Poway Unified School District Superintendent John Collins said in an online statement.
"This threat was taken seriously, acted upon seriously, and the consequences for the student will be appropriate," Collins wrote.