The “devious licks” TikTok “challenge” in September had kids around the U.S. to stealing soap dispensers off the walls of school bathrooms and committing other acts of vandalism, so school districts and police departments are increasing security in response to a rash of viral TikTok posts warning of school shootings and bomb threats at every school around the U.S. on Friday. 

“The social media threats had many educators on edge as they circulated in the aftermath of a deadly school shooting in Michigan, which has been followed by numerous copycat threats to schools elsewhere,” The Associated Press reports. The shooting at Michigan’s Oxford High School, which ended with four children dead, was the 28th school shooting this year. 

Law enforcement and schools have assured parents the vague threats aren’t considered credible, but at least one high school, in Gilroy, California, has canceled school Friday, starting the winter holiday a day early, out of an abundance of caution. 

“According to police, the original TikTok challenge was for students to skip school on Friday, Dec. 17 and at some point, the challenge was changed to a school shooting threat,” ABC 7News in Washington, D.C., reports. The threat originated in Arizona, according to Baltimore County Schools.

“We handle even rumored threats with utmost seriousness,” TikTok said in a statement on Twitter. “which is why we’re working with law enforcement to look into warnings about potential violence at schools even though we have not found evidence of such threats originating or spreading via TikTok.”

Thanks to a “safe harbor” provision in the 1996 Communications Decency Act, “it would be unlikely that TikTok would be liable if there were actually to be a shooting,” Jeff Kosseff, a cybersecurity expert at the U.S. Naval Academy, told AP. But that doesn’t mean TikTok can’t do anything, he said, noting that he also got a warning about this latest TikTok challenge. “They have a lot of flexibility to be doing the right thing and taking down harmful content. I am hopeful they are doing that.”