A photo of a gun was accompanied by the threat: "Dont come to Eastside tomrw."
Authorities initially didn't know which particular school the Tuesday post was intended for. Newark also has an East Side High School.
Law enforcement sources said it involved Paterson's school, originated in Virginia and was being thoroughly investigated. Severe consequences await whoever was responsible.
Newark school officials also were notified, they said.
Click here to follow Daily Voice Cresskill-Closter and receive free news updates.