Nationwide, schools have reported at least 56 such incidents since the February 14 shooting in Parkland, Florida, including threats via social media networks such as Snapchat and Instagram, according to CNN and affiliate reporting.
Some were reported in Florida, including in Broward County, where a gunman killed 17 people last week at Marjory Stoneman Douglas High School. But incidents happened in other states as well, including Texas, Virginia and California.
Though some turned out to be hoaxes, they prompted lockdowns and led classes to be canceled as a precaution.
Several reports in Houston area
In the Houston area alone, at least seven incidents involving a threat or a weapon have been reported since the Florida shooting, a CNN tally shows.
These include a school lockdown over a gun found in a 17-year-old student's jacket the morning afterward.
Authorities discovered the gun during a regular student screening at a Houston school, CNN affiliate KPRC reported.
In nearby Richmond, Texas, a 14-year-old student posted an image of a shotgun on Snapchat and warned students not to come to school the next day. The student was charged with making a terrorist threat, authorities said.
Another student, 15, was arrested Friday after he said on Snapchat, "You think you've seen a threat, I'll show you a threat." The message included a video of himself at a gun range shooting an AR-15
-- the same weapon used in the Parkland shooting.
The student was detained and charged with making a terroristic threat, the Fort Bend County, Texas, Sheriff's Office said.
"This is not a game or a way to get attention," said Fort Bend County Sheriff Troy Nehls. "I once again urge parents to have a conversation with their children regarding the seriousness of threatening posts."
Threats in Florida after mass shooting
In Broward County, a sixth-grader who threatened to kill students at a Lauderdale Lakes school this week was arrested and faces a felony charge of intimidation.
The 13-year-old said the post was a joke, the Broward County Sheriff's Office said, warning that those who make such threats will be prosecuted.
"Threats will be thoroughly investigated," it said.
Threats occurred elsewhere in the state, including in West Palm Beach. After an anonymous tip, police there arrested a student who brought two guns Thursday to Palm Beach Lakes Community High School. The school was put on lockdown, and the student was detained and faces expulsion, authorities said.
And in Gilchrist County, the school district closed Friday after receiving a threatening email the previous night.
Threats via social media
Some students have allegedly used social media to make threats. In Nutley, New Jersey, public schools closed Friday following a threat in a video posted to Instagram, police said.
The person identified in connection with the post is cooperating with the investigation, police said.
In Norfolk, Virginia, detectives detained a 14-year-old student Monday over threats to harm classmates if they attended school the next day.
Norfolk authorities said the threat was posted on social media.
The copycat phenomenon
In Fayetteville, Arkansas, police said a student allegedly made an online threat
Thursday to "shoot up the high school like they did in Florida." In Spartanburg County, South Carolina, a ninth-grader allegedly posted a photo of himself on Snapchat wearing a mask and showing an assault rifle with the words "round 2 of Florida tomorrow.
Both students were arrested.
Experts said such incidents are common after mass shootings, and authorities take them seriously, especially because some researchers believe mass killings can lead to copycat attacks.
The copycat phenomenon may be more acute in the United States because guns are more accessible.
"(Access to) firearms (is) a significant predictor of these incidents," Adam Lankford, an associate professor of criminal justice at the University of Alabama, said in a one study.