(CNN)Since the Parkland, Florida, school shooting on Valentine's Day, there have been at least 20 reports of other incidents involving a threat to a school or a weapon on a school campus.
Spartanburg, South Carolina
A ninth-grade student was arrested February 15 after posting a threat on Snapchat to students and staff at Broome High School, a spokeswoman for Spartanburg County School District Three said.
The student posted a photo showing an assault rifle with the words "round 2 of Florida tomorrow." The male student was charged with disturbing schools, the spokeswoman said, and the district has begun measures to keep him off school grounds.
A 13-year-old boy was arrested at Nichols Junior High School on February 15 after he told other students he wanted to shoot up the school and mentioned guns, Arlington police said.
Officers did not find a gun, but the student was charged with making a terroristic threat. Police said the student told them in an interview that he wanted to shoot the school with a Nerf gun.
A 16-year-old boy was arrested February 15 for possession of a handgun after a Plano West Senior High School resource officer investigated an anonymous Campus Crime Stoppers tip that a student brought a handgun to campus, Plano police said.
Sugar Hill, Georgia
A Lanier High School student faces criminal charges and disciplinary action after being found with a gun on campus February 15, Principal Christopher Martin wrote in a letter to parents.
A 16-year-old girl was taken into custody February 15 after police said she made "vague" threats on Snapchat toward Weatherford High School, police said. Officers took the girl to a juvenile detention facility on charges of making a terroristic threat. They did not find a weapon on the girl.
Daytona Beach, Florida
The Daytona Beach Police Department said on Facebook that officers took a 20-year-old man into custody after he threatened violence against his classmates at Mainland High School on February 15 and made "other disturbing general comments."
A 14-year-old boy was arrested February 15 after being found with a pellet gun at Loch Raven High School, the Baltimore County Police Department said. After a student told a school resource officer that the boy had a gun, the school was put on lockdown while police and tactical units searched the building.
Classes at the school resumed February 16.
Western Heights Public Schools said it received a threat involving its middle school February 15 from a student not at the campus.
The school was not placed on lockdown but was closed February 16.
West Palm Beach, Florida
Police in West Palm Beach, Florida, arrested a student who had brought two guns to Palm Beach Lakes High School on February 15. In a message sent to parents, Principal David Alfonso said the arrest came from an anonymous tip.
The student fled when officers approached to investigate. The school was on lock down as a precaution, Alfonso said in the message.
"The police recovered two guns in the same vicinity that the student fled. The student was arrested by our campus school police officers and now faces expulsion as defined in our student code of conduct."
Gilchrist County, Florida
After receiving a threatening email the night of February 15, the Gilchrist County School District decided to close February 16.
School officials said in a post on Facebook that they could not confirm the legitimacy of the threat.
Fair Haven, Vermont
Police in Fair Haven along with detectives from the Vermont State Police have charged 18-year-old Jack Sawyer with attempted aggravated murder, attempted first-degree murder and attempted aggravated assault with a deadly weapon, after threats to Fair Haven High School.
He was arrested on February 15, after two days of investigations.
According to a statement by Vermont State Police, Sawyer had "communicated his desire to cause mass casualties" at the school.
Nutley, New Jersey
Nutley Public Schools closed February 16 because of a threat in a video posted to Instagram, police said.
The video under investigation did not contain a direct threat to any Nutley Public Schools, and the person who was identified in connection with the post is cooperating with the investigation, police said.
Flower Mound, Texas
Flower Mound police said officers arrested a 16-year-old at Marcus High School on February 16 after officers found a small-caliber handgun in his backpack. The gun was unloaded but police said they found ammunition.
The student is facing felony charges for possessing a firearm in a prohibited place, police said.
Police then received notice of a rumor saying a student posted to social media after the initial arrest that he would be coming to Marcus High School at lunchtime to "finish the job." Police have not been able to find the source of the rumor, but they are still investigating with the FBI.
Abbeville, South Carolina
Law enforcement conducted a sweep of Abbeville High School after a bomb threat posted to Snapchat on February 16, the Abbeville County School District said.
A suspect was taken into custody and schools operated on a normal schedule February 16, the district said.
Police in Nicholasville, Kentucky, arrested two people on February 18 in connection with threats against Jessamine County Schools.
The two have been charged with terroristic threatening in the second degree, according to a statement by the Nicholasville Police Department.
"This was a situation where the public immediately did what they should have when they recognized a post on Snapchat that was a threat against Jessamine County Schools. The post was shared not only with law enforcement but with many others across the city, county and state. The information was crucial in developing the evidence needed to make the arrests," the statement said.
Norfolk, Virginia, detectives have arrested a 14-year-old Lake Taylor Middle School student on February 19 for threatening to harm classmates if they attended classes the next day.
According to a release by the City of Norfolk, the threat was posted on social media.
"During the investigation Norfolk Police identified the seventh grade student. He is facing a Class 6 felony for making threats of death or bodily injury to a person or persons on school property," the statement said.
Bullitt County, Kentucky
The Bullitt County Sheriff's Department has charged a juvenile in connection with threats of violence towards a school in the district.
In a statement on February 19, the Bullitt County Sheriff's Department said it "will be securing search warrants and working with the social media platform providers to identify the other hoax perpetrators in order to hold them accountable." The statement added "there is no threat against any of our schools."
Lexington County, Kentucky
The Lexington County Sheriff's department announced on February 19 that it was investigating " a potential threat made at Chapin High School."
The sheriff's department added it planned to place extra deputies on campus and increase patrols around the school on February 20.
Collier County, Florida
Two students were arrested at two schools in the Collier County Public School system on February 19, according to the Collier County Sheriff's Office.
An 18-year-old student was arrested at Palmetto Ridge High School after law enforcement and school investigators determined that he had a knife on school property. Acting on a tip about the student reportedly bringing a gas mask to school on February 15, investigators discovered concerning items in the student's backpack.
Officers found a note that said, "shoot up school," and "school shoot animae dead," along with a list of names of students. The list contained a drawing of a student with bullet holes to the chest and the words, "dead ha ha dead" written on the back of the paper. Officers also found a map of the Naples-area school, according to the press release.
A second Collier County student was arrested at Corkscrew Middle School, after a knife was discovered in his locker. The 13-year-old's locker was searched after school administrators and sheriff's deputies received a tip that the student had posted a photo of a weapon on social media over the weekend, according to the press release.
Broward County, Florida
A 13-year-old girl posted a threat on Instagram that students at Central Charter School would be killed this week, according to a Broward County Sheriff's Office news release. The threat was posted at 8:30 a.m. February 19. The sixth grader allegedly gave two dates: February 20 or 22.
The girl told deputies the threat was a joke, officials said. Her parents, the release said, were worried enough by the Instagram threat to keep their daughter home from school, before they knew she was accused of making the post. The student has been arrested and faces a felony charge of intimidation.