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 that sell school safety equipment. They have ramped up their marketing to school safety officials in the wake of 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 emailed pitches since the Parkland shooting. He said, “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 safe 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.