Paul Hankins keeps a box of smooth, colorful river stones — he calls them “fidgets” — in his classroom for students to hold when they need to soothe their nerves.

The stones also have a different purpose, as do the billiard balls and the plank from the old gym floor. They can all be used as weapons in an emergency.

“Before we go to the corner where everybody gets invisible, grab one of those,” he tells his students at Silver Creek High School in Sellersburg, Indiana. “We could cause one hell of a ruckus if we need to.”

Hankins does not begrudge teachers who argue that they should be allowed to carry guns to class, especially in the wake of attacks such as the Feb. 14 shooting that killed 17 at Marjory Stoneman Douglas High School in Parkland, Florida.

For him, stones are better.

They put the fight in “Run, hide, fight,” the mantra that he and countless other teachers are learning as lockdown drills become routine.

Yet experts are as divided as Hankins and his colleagues on how to protect students from violence or whether, for that matter, there is anything that can be done to prevent a determined attacker.

The latest shooting has intensified the debate over what tactics to use to protect against imminent danger: whether teachers should carry guns, or hide with their students; whether schools should invest in security devices such as door jammers or put more resources into crisis teams that could identify and intervene with troubled students. What follows is some of the discussion.

WHAT ABOUT TEACHERS CARRYING GUNS?

Most law enforcement experts argue that teachers should not carry guns. Civilians may be able to hit a bull’s-eye at the shooting range, but they lack the tactical knowledge of handling weapons that trained law enforcement personnel get.

Accidents happen. Guns can fall out of holsters, be taken from the classroom or accidentally discharge.

“You don’t want to have a gun that’s available to a student or another worker who may have mental health issues,” said Maureen S. Rush, vice president for public safety and superintendent of the police department at the University of Pennsylvania.

But some disagree.

Dave Workman, the senior editor of The Gun Mag and communications director of the Citizens Committee for the Right to Keep and Bear Arms, said arming guards or teachers could act as a deterrent so that no one had to draw a weapon in the first place.

“I understand the debate out there: ‘Should this be the job of a teacher?’” he said.

“Why not? The teacher is going to be there,” Workman said. “They become the first responder sometimes. It does take a while for police to respond to an incident.”

The Parkland school had an armed sheriff’s deputy who never encountered the gunman.

ARE METAL DETECTORS HELPFUL?

Metal detectors are unlikely to stop a gunman, experts say. But they can be useful in certain contexts, if, for instance, the school is in a neighborhood with high crime or gang activity, where students may try to bring guns or knives into school to defend themselves.

WHAT IS THE PURPOSE OF A LOCKDOWN DRILL?

In a lockdown drill, everyone in the school practices responding seamlessly to the presence of an intruder. Teachers and students go to a secure location, such as a classroom, closet or storage area that can be locked, and move out of sight, away from windows or doors.

Speed is important. The typical gunman is like water, the experts said, following the path of least resistance. Typically, a gunman will not try to kick in a closed door, but will look for one with a crack of light showing.

“Within 20 or 30 seconds, I, as a bad guy, should have very little easy access to anybody,” said David R. Connors, head of Connors Security Consulting Services in Spencerport, New York, and a former police officer. The locks or barricades on the doors do not need to be very strong, just to last until help can arrive.

The point of a lockdown drill is to know what to do automatically, without having to think.

“When something’s going down, you will not respond from your head, you will respond from your stomach,” Rush said. “You need to get that in your stomach. You need to know what instinctively you will do. That’s got to be part of your psyche. So it requires constant training.”

Different schools or districts may have different protocols. The important thing is that everyone, from teachers to students to parents, know what they are. “It has to be something that goes like clockwork,” said Dr. Irwin Redlener, a professor of pediatrics and public health and the director of the National Center for Disaster Preparedness at Columbia University.

IF SOMEONE KNOCKS ON A LOCKED DOOR, SHOULD THEY BE ALLOWED IN?

It may be heart-wrenching, but don’t open the door, experts say. A teacher has to think of the lives of the many who have moved to a safe place over the one person outside in the hallway who might bring danger in.

“You don’t open that door until you know the police are on the other side,” Rush said, adding “You’re doing it for the masses of kids.”

WHAT SHOULD STUDENTS BE TOLD?

It depends on their age.

“We don’t have to scare the heck out of these kids,” Connors said. “In a kindergarten through third-grade building, you might tell the kids, ‘Come over here and sit down next to the wall. Be nice and quiet. I’m going to read you a book.’ That’s all the kids have to know.”

Connors does not believe in making drills too realistic.

“You don’t have to have someone come over and try to kick in the door. I don’t think that’s constructive. With staff, yes. But not with kids.”

IS IT A GOOD IDEA TO LET STUDENTS USE CELLPHONES?

Phones should be put on silent, not even vibrating.

“Texting is fine,” Rush said. “You want to be invisible, and you want silence.”

Rush said she was alarmed by the students who took videos while hiding from the gunman in Parkland, not be because she disapproved of the video, but because it changed their mentality.

“That could be taking them out of their survivor mode,” she said.

WHAT ABOUT IDENTIFYING A TROUBLED STUDENT?

The experts vary on whether this is possible. Some say that students who become violent are often not the ones who seem most troubled or who make the most noise.

“The perpetrators are very often not the bullies or the guys roughing people up,” Connors said. “Quite often, it’s the guy who is quiet. In hindsight, they’ll go back and find that there’s some family or mental health issue.”

Others say that schools can do more to enlist students to help identify troubled classmates. Social media can be an early warning system, experts say. Students may express their violent thoughts there, as did Nikolas Cruz, who has been charged in the Florida shooting, according to law enforcement officials.

“These kids are begging us to stop them,” said Amy Klinger, a co-founder of the Educator’s School Safety Network, who consults on school safety from the perspective of the former teacher and principal that she is. “We just have to be better at picking up the warning signs.”

The schools alone cannot do it. Arguably, Klinger said, the failure to identify Cruz as troubled was a failure of the entire community, along with law enforcement agencies that had been warned about him.

“A school can identify the individual of concern, but they didn’t create that individual,” Klinger said. “So yeah, there is a role that the community and parents and our society has to play in identifying and dealing with those individuals as well.”

WHAT ABOUT INVESTING IN SECURITY DEVICES?

Law enforcement experts suggest hardening the perimeter, that is, putting up security cameras, door buzzers, gates and other barricades or high-tech devices.

But they say that buzzers are not enough, especially at arrival and dismissal times when hundreds of students may be milling around.

“After Sandy Hook, a lot of schools got very panicky and bought $5,000 buzzer systems,” Klinger said. “They bought the system that was breached by the intruder in the first 30 seconds of Sandy Hook. He shot out the door and walked into the school. So your buzzer system did absolutely no good.”

Rush said that everyone at the front office should be informed when a student has been suspended or expelled. A photo should be posted, and they should be told to call 911 if the person appears.

“This happens in corporate America all the time,” she said. “Workplace violence, where the person comes back and wants to kill the boss.”

Klinger said that schools should invest in teachers who mingle with students in the lobby or in hallways as a way to learn about troubling behavior that may be brewing.

Even in a large school, the experts said, doors need to be manned by someone who knows the students and will recognize anyone who looks out of place. Klinger said that this should be a teacher, who relates to the students, not a security guard.

“If you want to take this to the extreme, you can ensure the safety of kids by locking them in a cell, but that’s a prison,” Klinger said. “This is not a prison. It’s not a shopping mall.”

WHAT IF YOU CAN’T RUN OR HIDE?

Cultivate the survivor mentality, experts said. Fight.

“If he’s coming no matter what, if you got that one guy who starts breaking the door out, you take every object in the room and you beat the hell out of him to disarm him, because you’re going to die if you don’t,” Rush said. “So you might as well take your chances of trying to fight. We call it the survivor mentality.”

Rush gave the example of a professor who, as a gunman approached, took his phone out to look at pictures of his children one last time before he died.

“But a female student jumped and ran and locked the door. She had survivor instinct.”