The threat forced Coatesville Area schools to close early Thursday and hold classes virtually Friday.
Caln police said they were made aware of the "racially motivated threat" around 7 a.m., which claimed a shooting would take place next week at an unspecified school district building.
The teenager admitted to creating the post and told investigators it was intended as a "joke", according to local police.
The exact contents of the post were not made public by police.
The juvenile student is being charged criminally.
Police said this is a reminder of the importance of using social media responsibly.
"If you see a concerning post, such as in this case of school violence, please preserve the image or text and notify authorities right away in lieu of re-posting on social media outlets."
People can make reports by calling 911 or submitting a tip on the Safe2Say platform.
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