Students at Mira Costa High School were urged to leave their backpacks at home Thursday as the campus reopened after a two-day closure spurred by threats posted on Yik Yak.
The request was part of increased security measures by the Manhattan Beach Unified School District in response to threats posted on the social media network prompted school officials to close the campus for two days, said Carolyn Seaton, the district's executive director of human resources.
Any bag or backpacks brought onto campus were searched. Students' access to campus was limited to four entry points. School administrators warned that any students caught lingering on campus would be taken to the security office.
"We will continue to be watchful and cautious in our middle and elementary schools as well," district Supt. Michael Matthews said in a statement. "These procedures maximize the safety of our students and serve at the very least as excellent practice for future incidents in which safety is a concern."
On Wednesday, Manhattan Beach police said they had identified a teenage girl responsible for creating at least one of the posts on Yik Yak, a popular networking app on Apple and Android devices.
The girl is not a student in the district. She has been interviewed but is not in custody.
The high school was locked down Monday after a student notified a teacher about a "vague and nonspecific" message posted on Yik Yak.
The message read: "If you go to Costa, you should watch out very closely at school today."
Later that afternoon, a second message said, "nice try Costa, today was just a drill," Manhattan Beach police Officer Stephanie Martin said.
Officials said a third message appeared about 8 p.m.: "tomorrow at 2 be ready Costa, you're going on lockdown."
Yik Yak has come under fire after anonymous messages prompted similar school lockdowns and closures elsewhere in California and in other parts of the country.
School officials said police were working with Yik Yak to identify whoever sent the messages.
District officials have blocked access to Yik Yak on the school's Wi-Fi network.