It's now part of the pattern seen in the aftermath of school shootings like the one last week at Marjory Stoneman Douglas High School in Parkland, Fla., where a former student killed 17 people.
Schools elsewhere typically see a spate of similar threats says Rick Kaufman, who directs emergency management for the Bloomington school district.
"We call it the danger zone, and on average it's about 13 days. Within that time period you then will see a tendency of copycats," said Kaufman, who was in charge of communications at Columbine High School in Littleton, Colorado in 1999, when two students killed 13 people before killing themselves. Kaufman now advises schools around the country on crisis management.
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"By and large they are students that are acting out in anger," he said. "But they use the context of a school shooting as part of the narrative when they're acting out or they say something."
Only a small percentage of the threats come from students who actually plan to cause harm, Kaufman notes.
But schools have to take every message seriously.
Kaufman said to evaluate a threat, district leaders should look for a motive and whether the person in question has the means to carry out the action. Administrators should also look for other signs the would-be perpetrator may follow through.
"The more specificity the person has, the more you want to take that very serious," he said.
Officials at St. Paul's Cretin Derham-Hall high school faced such an evaluation this week after rumors surfaced of a threat Wednesday. School president Frank Miley says administrators immediately contacted the police and investigated until late Wednesday. They did not find a threat, and classes continued as usual Thursday.
Miley said safety is his first priority. At the same time, overreacting to threats can encourage more to crop up.
"Even in this highly charged, sensitive time, you have to determine whether the information you have is good. And the information we had was not good," Miley said.
Still, Miley said he encourages students to speak up if anything seems amiss. After so many high-profile incidents of school violence nationwide, Miley said many students do just that.
"They have a heightened sensitivity to school safety issues that I never had to live through and most of my generation never had to live through, but this is their reality, so they take it very seriously," he said.
Elsewhere in Minnesota, students raising the alarm prompted swift action. The Hill City School District in northern Minnesota cancelled classes Thursday after threats against the school and specific students on the messaging app SnapChat.
Superintendent Pat Rendle said school administrators and local law enforcement got a rush of texts Wednesday night from students sending screenshots of the threats.
"Our students did an excellent job of getting involved because ... we're only as safe as our students can allow us to be by talking about these things and reporting them to adults," she said.
The Aitkin County Sheriff's Department arrested two girls in connection with the Hill City incident.
The advent of social media has changed the landscape when it comes to school threats, Bloomington's Rick Kaufman said. He doesn't think overall threats toward schools have increased since then. But new online platforms give students a sense of anonymity.
"There's a false sense that you're not going to get caught," he said.
Kaufman said students may post messages that they don't think anyone will see. But online threats are also seen, reported and investigated. Those reports are key to keeping students and staff safe, Kaufman said.