A day after the school massacre in Texas, Ohio teacher Renee Coley thought her sixth-grade students would need time to process, so she opened class with a video about the news and started a discussion. Some students said they were sad. Some were dismayed the 19 slain children were so young.

After a few minutes, though, the conversation fizzled. Students were ready to move on with their day. To Coley, it was a grim reminder that the students had seen it all before, had grown accustomed to the ever-present threat of guns in school.

“They have no questions because these kids have grown up their entire lives and this has been the reality for them,” said Coley, who teaches in Reynoldsburg, outside Columbus. “They’ve processed this so many times. … It’s just another news day for them.”

The interaction highlights how students across America have grown up numb to the violence that has been playing out throughout their lives in schools and communities — and in much greater frequency since the pandemic.

The bloodbath at Robb Elementary in Uvalde, Texas, Tuesday marked the deadliest school shooting in the U.S. since the 2012 massacre at Sandy Hook Elementary in Newtown, Connecticut. Police say the shooter, an 18-year-old man, was killed by law enforcement at the school. Two teachers were also killed.

Although mass shootings of that magnitude are rare, researchers at the Naval Postgraduate School have recorded 504 cases of gun violence at elementary, middle and high schools since the start of 2020 — a number that eclipses the previous eight years combined.

The database includes a range of cases, including students brandishing guns or opening fire in classrooms, bathrooms, cafeterias or gyms. It counts students who have used guns to take their own lives at school. And it also tracks violence that doesn’t involve students, including overnight shootings near school grounds.

An alarming number have involved teens who turned to violence to resolve spur-of-the-moment conflicts, said David Riedman, a criminologist who co-founded the database at the Naval Postgraduate School’s Center for Homeland Defense and Security.

“The majority of those incidents are escalations of disputes,” Riedman said. “There are more teenagers carrying concealed handguns in school who are getting into fights and shooting people. And that is not something that we were seeing before the pandemic.”

The proliferation of guns in homes, coupled with an overburdened mental health system that has left many students without the help they need, has fueled the increase in school gun violence, researchers say.

In fact, violent incidents involving guns have increased across all of America since the pandemic started — not just in schools.

Schools are still among the safest places for children, emphasized Dewey Cornell, a psychologist and director of the Virginia Youth Violence Project at the University of Virginia, with most killings taking place in homes, public streets or other locales. But he also thinks mass shootings in schools will continue unless America addresses its longstanding shortage of school mental health workers.

“Some kids get helped, but a small number come away traumatized and scarred, angry and aggrieved,” he said. For some of those, “at some crisis point in their life, they are going to commit some type of violent act toward themselves or others.”

After every mass school shooting, Laurel Brooks, a high school graphic design and game-art teacher in Charlotte, North Carolina, tries to guide students through conversations and artwork that can help them express their thoughts. After the 2018 shooting in Parkland, Florida, that killed 14 students and three staff members, students worked on a graphic essay that described themselves as “the lockdown generation.” The theme has resonated with subsequent classes.

“It is frightening that it is consistent,” she said. “They have grown up with it. … They are still children, and they shouldn’t have to be resilient to this kind of trauma.”

Mass shootings in schools have remained a grim presence in America, but their numbers have held relatively even in recent years. Since 2012, a total of 73 students have been killed in school shootings with at least four victims shot and two victims killed, according to research by James Alan Fox, a criminologist at Northeastern University who studies mass killings.

Last year there was one school shooting of that scale, a rampage at an Oxford, Michigan, high school that left four students dead. In 2020, with many school buildings closed as part of pandemic precautions, there were no school shootings of that magnitude.