Matt Materazo’s kids are heading back to school this year with something new in their backpacks: an 18.5-ounce, quarter-inch-thick armor insert that will deflect a bullet from a .44 Magnum.

The owner of Gladiator Solutions, a ballistic-protection company in Danville, California, he designed the shield, called PakProtect, with the help of his two teenagers after last semester’s deadly school shootings in Parkland, Florida, and Santa Fe, Texas.

“I’m no different than any parent,” Mr. Materazo said. “I want my kids to be protected. I’m not happy about the reality that we have to do this, but it’s the reality of the world that we live in.”

His family isn’t the only one concerned about school safety. Across the nation, parents and students are taking extra precautions with bullet-resistant gear, while school districts are seeking to tighten their security, expand active-shooter training, and retrofit infrastructure to make it as gunman-proof as possible.

At Marjory Stoneman Douglas High School in Parkland, the 2018-19 academic year began Wednesday with $6.5 million worth of improved security, including surveillance cameras, a video intercom system at the only visitors’ entrance, and classroom doors that automatically lock when closed.

Only students and staff wearing ID badges will be allowed in through the reconfigured entrances at the beginning and end of the day, monitored by three school-resource officers, three security specialists, and a dozen campus monitors.

The enhanced safety measures have helped reassure nervous students. “We have better security now so I feel a little more at ease,” 16-year-old Anijah Avera told The Associated Press.

At the same time, plans for metal detectors have been postponed over the logistics of moving 3,300 students through them efficiently.

Max Schachter, who quit his job and founded Safe Schools for Alex after his 14-year-old son was killed in the Parkland shooting, praised the moves to shrink the entry points at the sprawling campus.

“You cannot have a major open campus like ours and be secure. And so that’s what we’re doing,” Mr. Schachter said. “We’re putting fencing up and funneling all the kids through three entrances.”

Mr. Schachter, who spoke Thursday before the Federal Commission on School Safety, emphasized the importance of making schools more difficult to penetrate with features such as bulletproof glass, panic buttons, and key fobs for teachers that can reach law enforcement with one push.

All those features and more have already been installed at an Indiana school known as the safest in the nation, which he said he toured with Broward County school officials.

“In the 19 years since the Columbine tragedy, we have focused most of our efforts on mental health and prevention. School hardening has been at the bottom of the list,” Mr. Schachter said. “Visiting that school in Indiana convinced me that it’s time to bring hardening up to the top.”

Homeland Security Secretary Kirstjen M. Nielsen cited her department’s $1.8 million grant offering for schools and other groups interested in training high-school students to “stabilize the injured and control severe bleeding” in the immediate aftermath of a tragedy.

“Helping students be able to be part of the solution after it’s safe for them to participate in the response is also important,” said Ms. Nielsen.

The commission, established by President Trump after the Feb. 14 Parkland shooting, is expected to release a report in the fall offering information on best practices and recommendations on school safety without imposing requirements.

“We know there’s no one size fits all plan, no standardized approach when it comes to school safety,” said Education Secretary Betsy DeVos, who chairs the panel. “Every school and every community is different. That’s why the commission will be making recommendations and not issuing mandates.”

In Texas, the state is expected to certify more staff to carry firearms on campus by adding as many as 165 school marshals. Already 71 school marshals have been trained and appointed, and another 94 are waiting to be approved or take the training, according to KUT News in Austin.

“It was slow at first, but especially after Santa Fe, it just pretty much exploded,” said Kim Vickers, executive director of the Texas Commission on Law Enforcement, which offers the training.

Students returning to class next week to Santa Fe High School can expect to see numerous changes. The building has been upgraded to accommodate four metal detectors and create a secure front entrance, with plans to install bulletproof glass in the front office.

Superintendent Leigh Wall released a video giving a tour of the infrastructure improvements. The district has also added five additional police officers, 10 civilian security assistants, and four trauma-trained counselors.

“They have been hired and will be ready to start at the beginning of school. They’re on board,” said Ms. Wall in the video.

A gunman killed 10 and wounded 13 in the May 18 shooting at Santa Fe, while 17 people were killed and 17 injured at Parkland. Both shooters, who were students, survived the attacks and have been charged with multiple counts of murder.

Mr. Materazo said he had been thinking for years about creating a bulletproof backpack product, but was reluctant to do so. That changed after the Parkland shooting.

“The idea of having to put a plate in my kids’ backpack and sending them off to a place where this kind of protection might be necessary was so strange to me. And emotionally, it was difficult,” he said. “But again, when Parkland happened, that’s when I said, you know what? This is the reality we live in.”

He said the product, decorated with a turtle logo and the slogan “We’ve got your back,” was launched Monday, and so far the response has been “amazing,” particularly from students less than thrilled about wearing one of the black nylon bulletproof backpacks now on the market.

“The reality is there isn’t a student in this country that wants one of those backpacks,” said Mr. Materazo. “Kids are very particular about what they wear. They want their Jansport, they want their NorthFace, they want their Patagonia.”

A portion of the sales from the shield, which retails for $129, are donated to the COR Foundation, which was formed after the Virginia Tech shooting in 2007 to “build a more compassionate culture” on campuses.

“It’s something I can do and we can do to protect our kids,” Mr. Materazo said.

•The Associated Press contributed to this article.