Students cross a street to enter for the first day of classes at Marjory Stoneman Douglas High School in Parkland, Florida, August 15, 2018.

Buying school supplies usually means checking off a list that includes notebook paper, No. 2 pencils, colored markers and a ruler. Now some parents are thinking about adding another item: bulletproof backpacks.

School shootings have become more frequent in recent years, leaving kids afraid and parents anxious. The violence has led to a surge in the number of companies selling products designed to help kids protect themselves better against handguns and some shotguns.

"I am considering buying a bulletproof backpack," said Nancy Landis, whose 15-year-old daughter goes to school in Los Angeles. "It's scary dropping my daughter off at school now with all the stuff happening. They're having lockdowns, police officers looking for things. It's happening so much."

For the first time, major retailers, including Walmart, Office Depot and Home Depot, are selling bulletproof backpacks or bullet-resistant plates that fit backpacks.

According to the Travel Goods Association, the U.S. backpack market last year was $3.7 billion, up nearly 10 percent from the prior year.

"Bulletproof backpacks have been the one that's been most in demand and most inquired about," said Yasir Sheikh, president of Guard Dog Security, a maker and distributor of self-defense products.

Sheikh said some kids might take the bullet-resistant inserts out of their own backpacks and then get "a false sense of security." Therefore, he said, Guard Dog Security decided it would only sell the bulletproof backpacks and not the separate bulletproof inserts.

More than a half-dozen manufacturers now make bulletproof backpacks for young people, ranging from elementary to high schoolers, with prices starting around $130 and going up to $400. The products also have caught on with college students.

Some of the backpacks are marketed as so-called "Level 3A" armor protection, a category certified to stop up to a .44 Magnum round but not one from an AR-15, the weapon used in February's Parkland, Florida, school shooting that killed 17 people.

"We've probably seen around a 200 percent to 300 percent increase since the Parkland shooting this year," said Joe Curran, founder of BulletBlocker, a Massachusetts-based bulletproof equipment maker.

Since the Columbine High School massacre in 1999, "more than 187,000 students have been exposed to gun violence" in schools, The Washington Post reported in March. That doesn't include the mass shooting in May that claimed the lives of 10 people at a high school in Santa Fe, Texas.

"Parents are grasping for any type of physical, tangible evidence of increased safety for their children," said Ken Trump, president of National School Safety and Security Services, an Ohio-based national firm specializing in school security. "Shootings are a high-impact but low-probability type of scenario."

More than half of U.S. teens are worried about the possibility of a shooting happening at their school, according to a Pew Research Center survey of 13- to 17-year-olds conducted after the Parkland massacre and released in April. It found 25 percent of teens were "very worried" and 32 percent were "somewhat worried."

"We hear from concerned parents that will actually call us," said Curran. "They are looking for some reassurance that they are doing the right thing buying a bulletproof backpack. They know it's kind of a scary thing to think about."

Several of the bulletproof backpack companies were started by people with law enforcement or military backgrounds.

"I got into it because I had two kids in middle school and [the 2007 shooting at] Virginia Tech just happened so I was a little freaked out," said Curran, an Army veteran who worked in law enforcement for 15 years before starting BulletBlocker in 2007. "I had some old bulletproof vests kicking around and cut them apart and put them in backpacks and told the kids: 'If a gunman comes in, hold the bag up between you and him and get out of there if you can.'"

California-based company Leatherback Gear sells a backpack that can convert into a bulletproof vest with two built-in armored panels for torso protection in the front and back. The company, which was started by two brothers in active-duty law enforcement, formally launched its first product in January after two years of development.

"We kind of liken it to a fire extinguisher," said Leatherback Gear co-founder Brad de Geus, who credits his brother for coming up with the product's design. "It's literally a reactionary tool to a life-threatening problem."

De Geus said the "run, hide, fight" strategy adopted by the U.S. Department of Homeland Security is "pretty much the standard" that has been taught to schoolchildren, teachers and business professionals of how to respond in a shooting.

"We want you to run away with front and back protection on and to hide with front and back protection on," de Geus said. "You need to survive that average window of 5 minutes until we [law enforcement] get there."

Some companies that originally sold armor to law enforcement or military markets have since expanded into the schools market with personal protection products.

"People are looking for a sense of security," said Emily Tunis, president and COO of Hardwire, a Maryland-based maker of bulletproof clipboards and backpack inserts for the kids and schools market. The company also makes bulletproof emergency shields the size of fire extinguishers for schools.

"Teachers and students have nothing to protect themselves right now," said Tunis. "So this is something that can help buy them a little bit of time until law enforcement gets there."

She said Hardwire began developing protection products for the kids and schools market after the Sandy Hook Elementary School shooting in Connecticut in 2012. The company previously designed and fitted helicopter and vehicle armor for military customers as well as for law enforcement applications.

Gladiator Solutions has been selling body armor for law enforcement and military markets for about four years but only recently expanded into the kids market with its PakProtect armor plate designed to fit inside backpacks.

"This isn't just about protection, it's about peace of mind," said Matt Materazo, CEO of the California-based company.

Materazo said he thought of doing the kids armor plate three years ago but didn't at the time "because it was a struggle for me emotionally [as a parent of two teenagers]. But it's the reality of the world we live in now."

However, parents like Landis concede that even bulletproof backpacks may not be enough to ease their fears.

"It's not 100 percent; it's just something to help," said Landis. "I also know my daughter doesn't always wear her backpack at school. That said, I still might invest in it just knowing it could make a difference."