The best way to shield a school from a gunman is to have a metal detector. Or doors that can be remotely locked. Or Twitter-trawling bots looking for threats. Or bulletproof clipboards, whiteboards and backpacks.

So says the fast-growing group of companies selling school safety equipment. They have ramped up marketing to school safety officials since the shooting last month at a high school in Parkland, Fla. But even as school districts rethink their security and seek to increase their budgets, they have little guidance from government agencies or independent consumer groups on which equipment would actually protect their students.

Lawrence Leon, the chief of school police at the Palm Beach County school district in Florida, said he had received thousands of e-mailed pitches since the Parkland shooting. “I’ve seen everything from door locks to apps to analytics to metal detectors, and I haven’t even gone through all of them yet.”

Schools were generally considered a haven from the outside world until 1999, when two students at Columbine High School in Colorado massacred a dozen students and a teacher. In late 2012, a gunman killed 20 first-graders and six adults at Sandy Hook Elementary School in Connecticut. Since then, more than 400 people have been shot in schools nationwide.

Campus security has become a growth market.

Last year, sales of security equipment and services to the education sector reached $2.7 billion, up from $2.5 billion in 2015, according to data from IHS Markit. After the Parkland shooting, demand is surging.

“Right now, there’s going to be a lot of appropriations dollars being sent to school districts without a lot of oversight,” said Curtis S. Lavarello, executive director of the School Safety Advocacy Council, a training provider. “There are no national standards in terms of products for school safety.”

Each July, the group holds a conference and expo about school safety that normally draws about 80 exhibitors and 700 guests. This year, after Parkland, registration is on track to exceed 120 companies and 1,000 visitors.

The annual school safety conference hosted by the National Association of School Resource Officers drew 622 participants in 2011, then 733 in 2014 and 923 last year.

Security options are manifold: palm scanners, mobile barricades, heat detectors, walkie-talkies, trauma kits, active shooter resistance training and more. In the fall, Florida Christian School in Miami began selling $120 ballistic panels for students to put in their backpacks. At a gun show in Tampa, Fla., last weekend, administrators and parents swarmed a booth offering similar panels for nearly $200 each.

The civilian body armor market was valued at $72.2 million in 2016 and is expected to more than double by 2024, according to Grand View Research. Richard Soloway, the chief executive of Napco Security Technologies, which makes safety software systems, said in an investor call Feb. 5 that campus safety was a “significant opportunity.”

But Heather L. Schwartz, who has studied safety technology for the Rand Corp., said that research into what actually works is “really thin.”

“There’s not a lot of evidence to help districts sort through the pile before investing in costly systems,” she said. “There’s a lot of hunger for some authoritative third-party source to go out and review these options.”