An unprecedented rash of threats phoned in to San Diego high schools Thursday kept more than 21,000 students in locked-down classrooms as police systematically swept 11 campuses but found no suspicious devices or people.

Over a two-hour period starting at 9:15 a.m., callers threatened shootings, bombings and other unspecified violence at campuses in two school districts stretching from San Ysidro to Mira Mesa.

Students at San Diego High School mill around after a lockdown that had kept them inside secured classrooms for about three hours Thursday. Lyndsay Winkley

Some students spent from 30 to 45 minutes crouched under desks, with windows and doors secured, while teams of police officers, some with dogs, searched school rooms, grounds and immediate neighborhoods. Strict lockdowns were then eased, but students at several schools remained indoors past noon while anxious parents waiting in parking lots fumed over restricted communications.

It was the most threats the San Diego Unified School District ever handled on a single day, said district spokeswoman Linda Zintz. Several times in the last school year, two or three campuses at a time – in San Diego and in other parts of the county – were locked down after threats were made over social media or by phone. On a single day in March, the district received an email of bomb threats to four campuses.

Affected San Diego Unified schools on Thursday were Crawford, Clairemont, Kearny, Scripps Ranch, San Diego, Patrick Henry, Mira Mesa, Morse, Point Loma and La Jolla highs. Serra High went on lockdown for about 45 minutes until officials sorted out that no threat had been made there.

Only Mission Bay, Lincoln, Madison, Hoover and University City high schools had no threats or lockdowns, but a school police officer was stationed at each one as a precaution, officials said.

Update This story has been corrected. Any earlier version omitted Hoover from the list of schools that did not receive a threat. There also was a threat made at Morse; an earlier version had it listed as both locked down and not locked down.

San Ysidro High in the Sweetwater Union High School District also received a threat and was locked down.

Learning took a back seat to high-security measures as the day unfolded on Thursday

“I think whenever you put students in lockdown, particularly at a high school where they are used to going from classroom to classroom, it definitely disrupts the learning process,” Zintz said.

“However ... our first priority is to keep our kids safe. Although it did disrupt some classroom learning, we are happy to report that everybody is safe.”

Ken Trump, a school safety expert with National School Safety and Security Services, an Ohio-based consulting firm, applauded the reactions in San Diego, saying they seemed to take the best-practices approach of evaluating the threat, locking the school down and checking the facility. He said evacuating students without a full assessment can actually put students in greater harm, depending on the situation, he said. He also cautioned against reacting based on emotion.

Trump’s consultant firm reviewed 812 school threats across the country from Aug. 1 to Dec. 31, 2014, and found a 158 percent increase since the previous year.

The study found that electronic devices and social media apps were fueling the growth. The study showed 37 percent of threats came from electronic sources.

The roughly three-hour lockdowns on Thursday tasked city and school district police. San Diego police Lt. Scott Wahl estimated from three to six San Diego police officers responded to each campus to assist schools police with searches and traffic control.

“It certainly was a draw on our staffing, and delays our response to other calls for service,” Wahl said. “(But) getting kids coordinated and protecting them is priority one. I don’t know how that could change.”

The first threat was phoned in to San Ysidro High School around 9:15 a.m., with the caller saying “something would happen” in the next 15 minutes, said district spokesman Manny Rubio.

Other threats started coming in about 9:30 a.m. Point Loma High received a phoned threat around 11:15 a.m. and La Jolla High School received a call less than five minutes later. Authorities did not say if the calls were believed made by a single person or a particular group, or what the motive may have been.

“You’re typically dealing with someone who is looking to disrupt the normal flow, looking for a little bit of a thrill,” said Mo Canady, executive director of National Association of School Resource Officers, based in Alabama. The organization puts on training for schools and law enforcement for these types of situations.

“I’m not aware of a modern-day case of someone calling the school announcing a threat and there actually being a threat or a device,” Canady said. “At the same time, you have to take it seriously. The last thing you want is to gloss over something like that and have something bad happen.”

Using an administrative lockdown that gives teachers and staff control over students but still doesn’t halt the school day can be an effective method for dealing with such threats, he said. “It allows teachers to keep teaching, tests to go on.”

Initially officials had teachers, students and staff shelter in place, but as more threats came in to more schools, they shifted to a more restrictive lock down as a precaution. No one was allowed to leave or enter while San Diego police and school district police walked through each campus.

At Scripps Ranch High, Principal Ann Menna sent an email to parents saying numerous schools received “hoax” bomb threats, and police had cleared her campus as safe by about 10 a.m. However the school remained on alert and Menna asked that parents not come to the school.

A city schools police officer at Mira Mesa High decided it was best to evacuate the campus and had students file out to playing fields around 11 a.m. However, students were sent back into classrooms a short time later until the lockdown ended.

Classes and lunch periods at the schools resumed around 12:30 p.m., with after-school activities going on as scheduled. However, many parents took their children out of school for the second part of the day.

One parent who was told she couldn’t take her child out of Kearny High School while the lockdown was still in effect reacted angrily, shouting an obscenity as she drove away.

Parent Delia Rohr got a voice-mail message about the lockdown at Kearny.

“They didn’t want us to come to the school but I came anyway,” Rohr said. “That’s my baby. I have to physically see him and make sure that… I mean. You know. People make mistakes all the time. It could be that they’re saying he’s fine but he’s not so I just want to make sure with own eyes.”

Teresa Mendoza’s son Matthew King, a freshman at Kearny, was about to leave his math class when a bell kept ringing.

“We sat down in our seats and they said it was a shelter-in-place,” King said. “Then later on our teacher said to get under our seats.” They remained crouched down in a classroom -- without air conditioning -- for 30 to 45 minutes.

About two dozen adults waited to pick up students outside San Diego High. Some said even though they knew the calls were likely hoaxes, they wanted to be sure.

“But it doesn’t make you feel safe when your kid’s at school and there’s a bomb threat,” said Angelica Arriaga, who was picking up her 9th-grade nephew.

Other parents were upset that they didn’t hear about the lockdown from the district sooner. Liliana Dalmorao (cq) said she heard about the lockdown from her 11th-grade daughter first.

“I came down here right away and I’ve been here ever since,” she said. “Some guy told me I could leave, that everything is OK, but I’m not leaving until I see my daughter safe.”

David Beltran, a junior, said he was in American History class when the lockdown announcement came over the loudspeaker.

“It gets everyone nervous, and you try and calm down, but it’s hard,” he said. He said his teacher put on a movie.

“What if something’s really happening out there? That’s the first thing that crossed my mind,” Beltran said.

At Serra High, where no threat actually had been received, the principal announced a lockdown over a loudspeaker.

Gregorio Gurrola left his job in Carlsbad when his daughter Citlalli texted him about the annoucement. “We were inside and then we had a message saying that we were going to be on lockdown. So we just went under our desks, covered the door, turned off the lights, and just stayed quiet,” Citlalli said.

“Kinda scary, huh?” Gurrola said as he and his daughter headed to their truck around 12:20 p.m.

Crawford High School Principal Richard Lawrence sent an automated message to parents’ phones when their lockdown lifted at 12:51 p.m. saying, in part:

“Ten San Diego Unified schools were placed in lockdown this morning due to received threats of violence. School police were either on campus or arrived within minutes of the threats. Campuses have been swept and determined to be safe. Police will remain on campus for the remainder of the day and will return tomorrow morning as a precaution.”

The district told parents via Twitter that they did not need to pick up their students from the affected campuses.

Trump, the schools security expert, said maybe 99 out of 100 bomb threats are unfounded, but no one can tell which one will be the real one. “You have to treat it seriously, that doesn’t equate to overreacting,” he said.

Trump encourages schools to put a communications strategy into place to connect with anxious parents during such situations, whether it be phone calls, social media or emails.

“I’ve sometimes seen the communications crisis become bigger than the security crisis,” Trump said.

The lockdown at the San Ysidro campus was lifted around 10:45 a.m. after the school was searched and nothing suspicious was found. School resource officers maintained a presence on the campus into the afternoon, Rubio said.