MOBILE, Ala. — Jarvis Murphy tried to get away.

He joined a fleeing crowd, heard gunshots echo on the stadium concourse, felt a bullet pierce the ground a few inches from his left shoe.

He kept running — toward the exit, toward his car, willing himself forward even as he got a jolt in one leg, then the other.

Outside the stadium, Mr. Murphy fell to the ground. When he rolled up his jeans, he found a gunshot wound below each knee. And the worst pain he had experienced in his 18 years.

“I was scared. I was really scared,” said Mr. Murphy, who was one of nine people shot in August after a high school football game in Mobile. He said he had never worried about someone bringing a gun to a game. “I’ll be cautious everywhere I go now. I’ll be scared something will happen.”

Mr. Murphy became a casualty that evening of an overlooked epidemic of school shootings — the kind that happens after class lets out, the kind that draws little attention despite a national push to fortify schools and protect children. Since mid-August, gunfire has erupted more than 20 times at or near school sporting events around the country, more shootings than took place during school hours. Since the start of 2013, at least 19 people have been killed and more than 100 wounded in shootings with some connection to school sporting events.

As gunmen have stormed into classroom after classroom during the school day, killing dozens, a stunned country has mobilized to tighten security, prepare children with elaborate drills and pass new legislation. But a pattern of after-school shootings, which have occurred this academic year at a rate of about one a week, has largely gone unnoticed. Sometimes, after linebackers and officials duck for cover from gunshots during a football game, teams even return to the field and go back to playing.

A database from the Naval Postgraduate School, where researchers have compiled the country’s most comprehensive list of school shootings, shows that shootings at school events are a longstanding problem. But efforts to prevent them have been halting, piecemeal and, in some cases, virtually nonexistent.

On some campuses where IDs are required to walk into school and active-shooter drills are standard, after-hours games are still wide open: Anyone who pays a few dollars for a ticket can stroll right in. In Mobile, where metal detectors might have alerted security staff to the gun used to wound Mr. Murphy, the school district only decided after the shooting to buy detectors.

“There’s a growing problem there and we know that it’s not some spurious thing — it’s something systematic,” said Justin Kurland, a researcher at the University of Southern Mississippi’s National Center for Spectator Sports Safety and Security. “How many of these have to happen before the schools and first responders start to take this more seriously?”

A Security Gap

The teachers and students of Jeannette High in western Pennsylvania knew exactly what to do if a gunman ever burst into a classroom. There was no plan, though, for what happened on Sept. 6.

With the Jeannette Jayhawks leading comfortably in the fourth quarter of a football game, Matthew Jones, the school superintendent, was chatting with fans between the press box and concession stand. Something crackled across his radio: gunfire at the stadium gate.

What happened next was a mix of confusion and improvisation.

As a spectator lay dying outside the gate, the public address announcer directed fans to evacuate. But many stayed put. Members of the marching band lay down in the bleachers.

“We train extensively, but we had trained for events that would take place inside the school or on the school grounds,” said Mr. Jones, who had fortified school buildings with grant money and participated in active-shooter drills. “What we had not really trained for was an open venue with patrons, with people from a different school district.”

Witnesses told the police they saw one man elbow another, who then pulled a gun and started shooting.

At many schools, an intensive focus on security during classroom hours has failed to carry over to after-school activities. That has left events without basic prevention measures and students and officials without plans for how to respond.

Since January 2013, there have been credible reports of gunfire at or around at least 108 school sporting events across the country, according to a New York Times review of incidents, many of which were tracked in the Naval Postgraduate School database. In addition to that database, The Times relied on interviews, police reports, court documents and local media accounts to track shootings.

Shootings can occur at all sorts of school events: proms, graduations, even a back-to-school night in Pennsylvania where an off-duty correctional officer accidentally shot himself in the parking lot. Most shootings at school activities were not meticulously planned massacres; instead, they frequently came after a disagreement or fight between a few people.

Basketball and football games, which often attract hundreds or even thousands of spectators, are especially vulnerable. Some of the very things that can make those events special — whole communities coming out to watch and longstanding rivalries between schools — can also make them dangerous.

“In an event like this where you expect people to come and have a good time and not have any issues, it’s just one of those things,” said Chief Ron Applin of the Atlanta Public Schools police. “Anybody could get a gun into the stadium, probably.”

In Atlanta, someone shot two boys, ages 12 and 15, just off campus after a preseason football game in August, leaving one of them paralyzed. Since then, Chief Applin’s officers, who already were stationed inside stadiums for games, focused more on the periphery of events, where people can linger without going through a metal detector.

Large-scale events are inherently difficult to police, but experts say there are ways to limit problems.

A 188-page best practices guide published by the National Center for Spectator Sports Safety and Security calls for security screening at every entrance and having the police hold drills on what to do if a gunman appears. It also suggests that schools consider installing metal detectors and limiting patrons to clear bags. Last summer, researchers released a free online training course, funded by the Department of Homeland Security, to teach police officers and educators how to limit security risks at after-school activities.

But in many cities, schools have waited until after a shooting to improve security. Dr. Kurland blamed limited resources — hiring off-duty police officers, buying metal detectors and developing training can cost thousands of dollars — and a baffling lack of awareness about the risks of school events.

“There’s been next to no attention given to thinking about this,” Dr. Kurland said, and “the consequence of that has been all these shootings, particularly over the last few years.”

A Distinct Epidemic

A preseason football jamboree in St. Louis ended in chaos. Fights broke out as crowds were leaving. Police officers called for backup, and it seemed to be over. Then someone started shooting.

Just up the street, four people were hit. Jurnee Thompson, an 8-year-old girl who had been watching football with her family, died. Jurnee had done well in school, her father told local reporters, and the trip to the football game was a reward for that. More than four months later, no one has been arrested and the police are still investigating.

Around the country, school-day shootings — seared into the national consciousness by massacres at Columbine in Colorado, Sandy Hook in Connecticut and Marjory Stoneman Douglas in Florida — led to a wave of activism and legislation that fortified school buildings and, in some places, tightened gun restrictions.

But shootings at school sporting events are often seen as spillover from daily gun violence in places like St. Louis, which has one of the country’s worst murder rates, and attract less attention and less media coverage.

Theories about the disconnect vary. One likely factor: It is rarer for gunmen to storm into schools during school hours, but those attacks have been far deadlier. Since the start of 2013, at least 50 people died in active-shooter incidents during the school day, compared with 19 killed at sporting events. None of the shootings at sporting events resulted in more than two deaths, while 17 people died in the Marjory Stoneman Douglas shooting alone in 2018.

Another reason for the difference in attention, experts suggested, is a numbness about gun violence in the places where sporting event shootings often occur. Many of the deadliest school-day shootings have been in suburban and rural areas, and the victims have frequently been white. Shootings at sporting events have happened in a variety of places, including large cities, and many of the victims have been black.

Michael-Sean Spence, the director of policy and implementation at Everytown for Gun Safety, a group that supports more gun restrictions, lamented “a barrier that prevents people from seeing the residents of these communities as whole individuals deserving of public safety, and because of that the coverage has been skewed.” He added, “It has been underreported.”

Federal researchers estimated that about 1,800 children were killed by gunfire in the United States in 2017, and that thousands more were wounded. School sports shootings have been reported in 36 states since 2013, but three-quarters of the incidents have happened in the Midwest and the South.

In the most notorious school-day shootings, attackers made detailed plans for their massacres. But in many shootings at after-school activities, the violence was more spontaneous, a consequence of a fight or teenage dispute that escalated.

“We don’t have anything to suggest there was any planning about, ‘I’m going to go here and I’m going to shoot up and I’m going to get all these people,’” said Lawrence Battiste, the police chief in Mobile, where two people were shot outside a high school basketball game seven months before the shooting at the football stadium. “What we have is reckless behavior by adolescents that had access to weapons.”

A Pain That Lingers

Nobody was struck by the gunfire at Woodward High’s homecoming game. But what happened that night in Toledo, Ohio — the fleeing from the bleachers, the jumping over fences, the hiding in the school cafeteria while police officers swarmed — took a toll.

At the homecoming dance a day later, some students mistook the sound of popping balloons for more gunshots. When class resumed on Monday, some students stayed home. And, perhaps most painfully, students said they sensed that some in Toledo had dismissed Woodward as a place where violence was expected, even tolerated.

“We wanted to be the ones to say, ‘O.K., we’re not going to accept this, we’re going to take this farther and we’re going to keep going until something changes,’” said Jonnay Flenoy, 16, who along with other Woodward students organized a news conference to denounce the violence and describe how it had affected them.

For Mr. Murphy, the teenager shot in both legs in Mobile, normalcy might never fully resume. His leg brace is now off, but scars and pain remain. He no longer plays pickup basketball. He says his demeanor is forever changed.

“I’ll just be nervous, I’ll be watching people,” said Mr. Murphy, who graduated from high school in May. “If I go in Walmart, that’s one place I’ll be cautious at, too. There’s a lot of folks in there. There’s no telling what type of weapons someone has got.”

The police in Mobile made an arrest hours after the shooting at the football game. Chief Battiste said someone had pulled a gun during an argument and started firing indiscriminately. Later, the police said they found evidence that a second person may have shot a gun, too.

Mr. Murphy, who hopes to eventually attend college and perhaps become an electrician, does not talk about the shooting much. But it is never far from his mind.

He wonders whether working a longer shift at McDonald’s that night, even one more hour, might have kept him away from the violence. He recalls the agony of waiting for an ambulance that seemed to never arrive. And, most vividly, he describes the sound — how each bullet reverberated on the tunnel-like concourse as if it were being fired repeatedly and from every direction.

“I’m scared to go back in the stadium,” he said. “I’m scared to go to a football game, period.”