Emergency personnel respond to a shooting inside North Park School Elementary School on April 10 in San Bernardino, Calif. (Rick Sforza/Los Angeles Daily News via AP)

Two adults and two other people, possibly children, were shot today at an elementary school in San Bernardino, Calif., in what authorities say they believe may have been a murder-suicide.

It's the 12th school shooting of 2017, according to a running tally kept by Everytown for Gun Safety, a pro-gun-control group that sprang up in the wake of the Sandy Hook shooting in Newtown, Conn.

Everytown's criteria for inclusion in its count are broad. They include shootings that happen at colleges and universities, as well as at high schools and elementary schools. Accidental discharges — when a gun inadvertently goes off in someone's pocket, for instance — are included as well.

Still, the tally gives some sense of the ubiquity of guns and gun violence, intentional or otherwise, on school campuses across the United States. The San Bernardino school shooting is the second in the past three days: on Friday, a student was accidentally shot at a college in St. Paul, Minn., after another student inadvertently fired a handgun through a dorm room wall.

Earlier this year a Kansas State University student accidentally shot himself in the leg in a dorm room. Five days before that, in Florida, a parent picking up a child from elementary school accidentally fire a gun in his car. One day earlier, a middle school student in Alabama was showing a classmate a handgun in his backpack when he accidentally pulled the trigger, firing a round into the floor.

Earlier this year in Ohio, a 16-year-old high school student walked into school and shot a classmate with a shotgun. After a basketball game in South Carolina, an unidentified individual shot into a crowd leaving the gymnasium, wounding three.

If confirmed, the San Bernardino shooting would be the first this year in which an individual was shot and killed on school property by another person.

Since 2013, according to Everytown, there have been about 200 of these school shootings in the United States -- about one a week. In recent years, lawmakers in numerous states have been busy making it easier for students to carry guns on college campuses. Nine states have “campus carry” laws on the books.

Supporters say the laws help students defend themselves in the event of a mass shooting or other violent incident. But a recent literature review by researchers at Johns Hopkins University found little evidence that concealed carry laws have any effect on mass shooting trends. In fact, they found, the "best available evaluations of these policies indicate that these right-to-carry laws increase violence."

More guns, more crime, in other words — at school or anywhere else.