A teenage boy who screamed threats to 'slaughter the Christians' outside a school in Sydney's west has been charged with intimidation and offensive conduct.

Police allege the 14-year-old boy from Wentworthville was sitting in the passenger seat of a car when he yelled the abusive comments while waving a flag outside Maronite College of the Holy Family in Harris Park last Tuesday.

Onlookers were left shaken and terrified after a nun told police the boy was waving a flag that resembled the IS flag and had threatened to "slaughter the Christians."

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