Anonymous threats made on the social media app TikTok have put U.S. school districts and parents on high alert.

The threats seem to be part of a viral challenge, which claims that multiple school shootings will occur on Friday, Dec. 17.

As a result, many schools around the country have strengthened security and issued warning letters to parents. According to the districts, none of the threats are deemed to be real. Still, classes at Gilroy High School, for example, have been canceled for Friday “out of an abundance of caution,” Gilroy police said.

“Law enforcement agencies have investigated this threat and determined that it originated in Arizona and is not credible,” the official Baltimore County Public Schools Twitter account tweeted Thursday.

Authorities have informed TikTok, which has a security team to deal with the issue. Parents are also encouraged to talk to their children about the matter and ask them to report any suspicious behavior, the Santa Clara County Office of Education stated.

In a letter obtained by Fox 6 Milwaukee, the deputy superintendent of Wisconsin’s Waukesha School District wrote. “Parents, we are asking you to discuss this with your children, we take all threats seriously and any threats against the school or district will be disciplined to the greatest extent possible, up to and including expulsion from school.”

Although the TikTok threats are not credible, schools are taking additional safeguards.

The Scottsdale Unified School District sent a letter to parents emphasizing that it “takes all threats seriously.”

“We are in constant contact with our local law enforcement partners, who inform us these are NOT credible threats; however, you may notice an increased police presence on and around our campuses today and tomorrow,” the letter reads.

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