Why did the numbers change?
It all comes down to how Everytown, an advocacy group that supports stricter gun laws, defines school shootings.
According to Everytown, "any time a firearm discharges a live round inside a school building or on a school campus or grounds," it counts as a school shooting, regardless of whether or not the shooting results in injury or death.
For example, a third-grader at Harmony Learning Center in Maplewood, Minnesota, pressed the trigger on a school liaison officer's gun on Feb. 5. The weapon discharged and hit the floor — no injuries were reported.
Is that a school shooting? By Everytown's definition, that's exactly what it is.
"Every time gunfire breaks out on school grounds, it can shatter a child's sense that they are safe in their school and in their community. Tracking each of these incidents is an important way to measure some of the many ways that shootings affect children in this country," Sarah Tofte, director of research and implementation at Everytown, told CNBC in an email.
The Washington Post challenged this definition in an article published Thursday, saying Everytown had inflated its numbers "by including incidents of gun fire that were not really school shootings."
The Post pointed to one incident in particular. On Jan. 3, a 31-year-old man committed suicide outside of an elementary school in Michigan. Everytown counted the suicide as a school shooting. The elementary school, however, had been closed since June – seven months. No children or teachers were present at the closed school.
After the Post published its piece, Everytown removed the Jan. 3 suicide from its tally of school shootings. A spokesperson from the organization later offered this explanation:
"When we initially tracked this tragic shooting, news reports did not make it clear that the school was shuttered or not active. After we reviewed later stories, it became clear that the school had been closed for at least several months at the time of the incident. This does not meet our definition and led us to removing it from our database," Kate Folmar, communications director at Everytown told CNBC in an email.
Here is a list of all 18 shootings included in Everytown's original tally, ranked from most to least severe:
Feb. 14, 2018 - Parkland, Florida
A 19-year-old former student entered Marjory Stoneman Douglas High School and opened fire, killing 17 and injuring multiple others. The school was put into lockdown. The incident has been ranked as one of the top 10 deadliest mass shootings in modern U.S. history.
Jan. 23, 2018 - Benton, Kentucky
A 15-year-old student brought a handgun to Marshall County High School and opened fire in the school's atrium, leaving two dead and 17 injured, the Associated Press reported.
Jan. 31, 2018 – Philadelphia, PA
A 32-year-old man was shot and killed outside of Lincoln High School during a basketball game. Police responded to reports of a fight, and said at least three different weapons were discharged. The school was put into lockdown, according to the AP.
Jan. 22, 2018 – Italy, Texas
A 16-year-old student at Italy High School opened fire with a semi-automatic handgun in the school cafeteria. The gunman wounded a fellow student, who later recovered, the AP reported. The school was placed under lockdown, following the incident.
Jan. 20, 2018 – Winston-Salem, NC
A 21-year-old football player, Najee Ali Baker, was shot and critically injured following an altercation at a Wake Forest University party. Baker was taken to a hospital, where he succumbed to his injury, according to WTHI-TV.
Jan. 10, 2018 - Sierra Vista, Arizona
A 14-year-old student at Coronado Elementary School shot himself in the bathroom at school. The shooting was initially reported as an active shooter, forcing the school into lockdown. The seventh grader was pronounced dead at the scene, USA Today reported.
Feb. 5, 2018 – Oxon Hill, Maryland
An Oxon Hill High School junior was shot twice in a school parking lot, during what police say was an attempted robbery. The 17-year-old later recovered. The shooting took place on school grounds, but after school hours, AP reported.
Feb. 1, 2018 - Los Angeles, California
A 12-year-old student at Salvador B. Castro Middle School was charged with negligent discharge of a firearm after a semi-automatic rifle she brought to school went off. Four students were injured, and the school was placed on lockdown for several hours, USA Today reported.
Feb. 8, 2018 - New York, New York
A 17 year-old student fired a gun, hitting the floor of a classroom at Metropolitan High School. Police took the student in custody, but no injuries were reported, ABC7 reported.
Jan. 22, 2018 – Gentilly, Louisiana
An unknown person in a pickup truck drove past The NET Charter High School and fired shots at students standing in the campus parking lot. One boy was injured, but not by gunfire. Two students were arrested on suspicion and the school was temporarily placed into lockdown, according to the The Times-Picayune.
Jan. 15, 2018 - Marshall, Texas
Gunshots were fired from a vehicle in the parking lot of a dorm at Wiley College. A bullet was found to have gone through a window into a dorm room, but none of the residents were injured.
Jan. 10, 2018 - San Bernadino, California
Gunshots came through a window at California State University. The university immediately went into lockdown. No suspects were identified and no injuries were reported, AP reported.
Jan. 4, 2018 - Seattle, Washington
Jan. 26, 2018 - Dearborn, Michigan
A fight broke out during a basketball game at Dearborn High School. School officials removed the two persons involved, who were not students. Shots were later fired in the parking lot, but no injuries were reported, according to WXYZ Detroit.
Jan. 25, 2018 - Mobile, Alabama
A disagreement between two 16-year-old students at Murphy High School escalated when one of them pulled out a handgun. The student with the handgun then fled school administrators, who tried to calm him, firing four or five times into the air. Nobody was injured, according to Fox 10. The suspect was taken into custody and charged for multiple offenses, including possession of a weapon on school property.
Feb. 5, 2018 - Maplewood, MN
A third-grader at Harmony Learning Center pressed the trigger on a school liaison officer's gun. Although the weapon was outfitted with a trigger guard, it discharged and hit the floor. No injuries were reported.
Jan. 10, 2018 - Denison, Texas
A student at Greyson College picked up a gun, belonging to an advisor. She discharged the weapon, believing it was not loaded and shot through the wall. There were no injuries and no charges filed, KXII reported.
January 3, 2018 - St. Johns, Michigan
A 31-year-old man died by suicide in the parking lot of East Olive Elementary School. There were no students or staff at the scene, because the school had been closed for seven months, WILX reported. This is the incident Everytown decided to remove from their list of school shootings.