An Old Field Middle School official in Greenlawn notified police about the threat Friday afternoon, May 17.
The school searched the teen's locker, Suffolk County Police said.
She was arrested at the school at approximately 4:30 p.m. and charged with making a threat of mass harm, a Class B misdemeanor.
Her name has not been released due to her status as a minor.
She will be arraigned in Suffolk County Family Court at a later date.
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