The day after a gunman fatally shot 17 people at a South Florida high school, many of America's teenagers went to school, as always, returning to routines that usually felt safe.

But on campuses across the country, a palpable uneasiness descended on faculty, staff and students, who struggled with news out of Parkland, Florida, where police say Nikolas Cruz walked into Marjory Stoneman Douglas High School with an AR-15 and opened fire.

Adding to the anxiety: The spread of copycat threats on Snapchat and other social networks, where ominous messages proliferated.

Though most appeared to be hoaxes, administrators, police and school resources officers were on high alert, looking out - warily - for students motivated to threaten or engage in similar behavior.

In Arkansas, a Fayetteville High School student allegedly threatened to "shoot up the high school like they did in Florida" - and was arrested. "The police determined that the student did not intend to carry out the threat, but the act of making a threat against a school is against the law," officials said in a statement early Friday.

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In Spartanburg, South Carolina, a ninth-grade student at Broome High School was arrested after he allegedly posted a photo of himself on Snapchat wearing a partial mask and holding what appeared to be an assault rifle.

The photo was captioned, "Round 2 of Florida tomorrow," Spartanburg sheriff's officials told Fox affiliate WHNS.

Authorities told the station that a parent of another child had reported the threat and helped deputies identify the student in the photo.

When confronted by deputies, the student in the Snapchat photo said he'd posted it in jest.

"After informing the suspect and his parents of the law violation, the suspect stated his Snapchat post was just intended as a joke, and that he didn't have any serious intentions," Spartanburg County Sheriff's Lt. Kevin Bobo told WHNS.

Spartanburg County School District 3 officials weren't taking any chances, saying in a statement that they were providing additional security at the school.

"The safety of our students and staff is of paramount importance," the district's statement said.

The Snapchat post spread well beyond South Carolina; the Broward Sheriff's Office, in the Florida county where the deadly Parkland shooting took place, said late Thursday that "variations of the post have continued to be circulated over social media with captions added to warn people not to go to various schools throughout South Florida."

As Broward officials were publishing their post on Facebook, Florida's Gilchrist County School District announced that it was canceling classes Friday due to "an email threat."

"In light of the recent incident in Broward County, and out of abundance of caution, we will be closing school tomorrow, February 16th in order to ensure the safety of our students and staff," the district said.

Two hours later, another district, in New Jersey, announced that schools would be closed Friday, due to "a security threat."

"As both the Superintendent of the Nutley Public Schools and as a parent, and because of the nature of the world in which we live, there was no other decision to be made," Nutley Public Schools Superintendent Julie Glazer said on Facebook.

On Friday morning, reports of threats continued to emerge - including one warning high schoolers in Massachusetts of a "Florida pt 2." Other threats prompted three school lockdowns in North Carolina and an investigation at a middle school in Georgia.

After the threat - and arrest - in Fayetteville, Arkansas, where a student had allegedly posted about plans to "shoot up the high school like they did in Florida," Fayetteville Public Schools Superintendent Matthew Wendt said Friday morning: "We look forward to a regular day of school today."

In the aftermath of past mass shootings - including the one in 2012 at Sandy Hook Elementary School in Newtown, Connecticut - schools nationwide will feel "ultracautious" for an average of 10 to 14 days, said Mary Ellen O'Toole, a former FBI profiler. She said the increase of threats and false alarms in the days after a national tragedy is a noticeable phenomenon.

"There are certain things that occur in our culture that can provoke the desire to engage in similar behavior," she said. "We know the crime of mass shootings, especially like this one, can provoke someone who's already considering it."

Administrators and police can't look at a threatening post - such as the one from the Broome High School student - the day after a mass shooting and say, "Aw, there's no way he's going to do that," O'Toole said.

She said the rise of social media use among teenagers also contributes to the number of copycat threats, in part because the person posting the threatening message isn't able to witness the emotions of those who see it.

"When you do it behind a computer screen, you don't see that at the other end, you've put somebody in tears," O'Toole said.

It's also likely that children don't realize the seriousness and finality of school shootings, she said.

That could be the case with a sixth-grader at Nova Middle School in Broward County, Florida - also home to Marjory Stoneman Douglas High School - who was arrested Thursday for writing a note threatening to shoot up her school.

"I will bring a GUN to school to kill all of you ugly a - kids and teachers," the 11-year-old allegedly wrote, according to NBC Miami. "I will bring the gun Feb. 16, 18. BE prepared."

The student allegedly slipped the note under the assistant principal's office door, NBC Miami reported. She later gave a written confession to administrators.

Another Broward County school, North County Preparatory School, was put on lockdown after a teacher sent a text message that said she thought she heard shots, according to WSVN. Police went classroom to classroom conducting a search, then lifted the lockdown and reported a false alarm. While responding to the incident, a Broward Sheriff's Office deputy accidentally shot himself in the leg, according to WPLG.

School officials said a student at Sandy Springs Middle School, near Atlanta, posted a threat against his school on social media, which also made its way to students at the nearby high school, according to NBC Affiliate WXIA.

Authorities did not describe the threat but said that the middle-school student was taken into custody.

In Brooklyn, two 16-year-old boys threatened to shoot up their school less than two hours after news broke of the South Florida shooting, the New York Daily News reported. About 4 p.m. Wednesday, the teenagers posted two photos online: One showed a boy holding a rifle, with the caption, "We're gunning down tmrw," while the other showed a boy in a black ski mask, with a caption that read, "Don't come to school tomorrow." In the second photo, two fire emoji replaced the boy's eyes.

One boy was arrested at his home Thursday and the other turned himself in, according to the Daily News.

In Hamilton, Ohio, police arrested a student at Ross High School who sent "a post on social media referring to the recent school shooting in Florida," Ross Township police said in a statement. He faces a felony charge of inducing panic and was being held at the Butler County Juvenile detention center, police said.

And students in southeastern Massachusetts were warned in an anonymous social media post not to go to school Friday at B.M.C. Durfee High School because it would be "Florida pt 2," according to the Providence Journal.

The school was open Friday morning, though officials said extra security measures had been put in place.

"It's always a little nerve-wracking as a parent to get a call like that," Melissa Panchley, a parent and former school committee member, told the newspaper. "But we need to continue to live the way we live. It is really sad this is the society we live in today. It's not surprising. That's sad that it isn't surprising."

Other incidents involving guns and schools Thursday include:

- The arrest of a 13-year-old at Nichols Junior High School in Arlington, Texas, who allegedly threatened to shoot up his school with an AK-47 assault rifle, according to Fox 4 News.

- The arrest of two students at Palm Beach Lakes High School in West Palm Beach, Florida, who allegedly brought guns to school, according to the Palm Beach Post.

- The arrest of a male student at Clarksburg High School in Clarksburg, Maryland, who allegedly brought a loaded handgun to school, according to Montgomery County police.

- The arrest of a 16-year-old student at Marcus High School in Flower Mound, Texas, who allegedly brought a weapon to school, according to school officials.

- The arrest of a student at South Garland High School in Garland, Texas, who allegedly brought a gun to school that was not loaded, according to WFAA.

- The arrest of a Plano West High School student in Plano, Texas, who allegedly brought a handgun to school that was not loaded, according to WFAA.

- The arrest of a student at Lee's Summit North High School in Lee's Summit, Missouri, who allegedly brought a gun to school that was not loaded, according to KMBC.