The incident happened in Lake Ronkonkoma, at Samoset Middle School, at around 1 p.m. Friday, Sept. 13.
Suffolk County Police said a staff member asked the teen to empty his bag for an unrelated incident when the brass knuckles were discovered.
The student was arrested on suspicion of criminal possession of a weapon, a misdemeanor. He is expected to be arraigned in Suffolk County Court on a later date.
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