Nassau County police say Anthony Larosa was arrested shortly after midnight Thursday after Douglas McArthur High School officials in Levittown were notified of the graffiti at the Town of Hempstead Park.
Police say it stated: "I am going to shoot up McArthur on Friday 10/15/10" and "This is not a joke."
Police did not know how long it had been there.
Larosa, of Levittown, was charged with making a terrorist threat, criminal mischief and making graffiti. He is set to be arraigned Thursday. It was not immediately known if he was a student at the school or whether he had retained an attorney.