In schools across the nation, students prepare for the worst-case scenario: a gunman firing on them and their classmates. As part of safety drills, many are learning how to hide in closets with their teachers, how to keep quiet and how to barricade doors — anything to increase their chances of survival.

Now, a leading gun-control advocacy group and the nation’s two largest teacher unions are calling on schools to halt the most extreme active-shooter drills, such as those that occur without advanced warning and include simulated gunfire. For other kinds of lockdown drills, the groups laid out guidelines they say could minimize trauma for students, while emphasizing that little proof exists the drills make students safer in a shooting.

In a white paper published Tuesday, Everytown for Gun Safety, the American Federation of Teachers and the National Education Association cited mounting anecdotal evidence that the drills, while intended to keep students safe, are inflicting trauma and leaving children anxious, rattled and unable to focus in the classroom. Researchers who have begun to study the problem are making similar findings.

“There’s very little data that shows that these drills are effective,” said Shannon Watts, founder of Moms Demand Action, which advocates for strong gun restrictions and is part of Everytown. “There is data that shows that they cause trauma and anxiety in kids.”

The percentage of schools that had lockdown drills has more than doubled in the past 15 years, according to the National Center for Education Statistics. In the 2005-2006 school year, 40 percent of schools reported holding lockdown drills. A decade later, the percentage jumped to 95 percent.

In the policy paper, the groups examined media coverage of school lockdowns that illuminated cases in which police used pellet guns and had someone play the role of an attacker in a mask to simulate a school shooting. They also relied on research from criminal justice experts and child psychologists, and data from law enforcement on school shootings to make their case.

Many states, including Florida, have passed laws requiring schools to have lockdown or active-shooter drills. Following the deaths of 17 people in a shooting at Marjory Stoneman Douglas High in Parkland, Fla., the state began requiring lockdown drills monthly.

Elizabeth Brown became principal of Forest High in Ocala, Fla., not long after authorities say a former student walked onto campus and shot a classmate in 2018. The violence unfolded two months after the shooting at Parkland and on the anniversary of the shooting at Columbine High in Colorado, where a pair of students killed 13 people in 1999.

Brown gives parents notice before a drill happens, and students learn as soon as it starts when she comes on the public-address system and says, “This is a drill. This is a drill. Remember, students and teachers, this is a drill.”

Last month, when the school had to go on an actual lockdown, several students asked to see counselors, overwhelming the school’s small staff. School leaders “need to know that [lockdowns] may trigger some feelings and emotions that [students] may not even know that they had,” Brown said.

Sarah Lerner, a teacher at Marjory Stoneman Douglas High, said the first drills after the 2018 shooting left some students shaking and in tears. She said she believes monthly drills are excessive and that she would leave the profession if forced to participate in an active-shooter drill that involved firing simulated weapons — which has happened at some schools.

“It’s just demoralizing, dehumanizing,” Lerner said.

In some places, schools have gone to extremes to prepare students and teachers for the remote possibility of an active shooter on campus.

In Indiana, police officers ordered teachers during an active-shooter drill to crouch down while officers fired pellet guns at them, leaving some of them bloodied and bruised, according to a state employee’s testimony at a legislative hearing. It prompted calls for legislation to bar police from shooting airsoft guns — which use projectiles made from plastic or similar material — at teachers during safety drills. At a Florida high school, an unannounced active-shooter drill sparked chaos and left students with nightmares.

Randi Weingarten, president of the American Federation of Teachers, said she kept hearing anecdotes about active-shooter drills leaving those they are intended to protect traumatized and sometimes even physically wounded. She said those kinds of stories moved the union to put out the white paper.

“The cure is becoming worse than the problem we are trying to solve,” Weingarten said.

Real lockdowns are often triggered by threats of violence — such as those communicated over social media — and violence that occurs close enough to a school campus that worried administrators choose to play it safe. An analysis by The Washington Post showed that more than 4 million students endured lockdowns during the 2017-2018 school year.

Ryan Pascal, a senior at Palos Verdes High near Los Angeles, said one of the drills she can recall left her feeling more unsettled than prepared. The 17-year-old, who is on the national advisory board of the gun-control advocacy group Students Demand Action, also part of Everytown, said chaos reigned and students got mixed messages about what was happening — and even whether it was a drill. At one point, students were ducking under desks. Then, they were marched out to the school football field.

She knew it wasn’t real, but it still left her anxious.

“There always is that tinge of fear for me that something is wrong or something could go wrong,” Pascal said.