Second arrest after IS threats in NSW

A second teenager has been charged after death threats were allegedly screamed at Christians outside a school in Sydney's west.

A 14-year-old was in the front passenger seat of a red hatchback when he allegedly began yelling abuse outside the Maronite College of the Holy Family in Harris Park on September 16.

Onlookers told AAP the boy and the car's driver threatened to "kill the Christians" and slaughter their children while brandishing an Islamic State flag out the window.

Police said an 18-year-old was arrested on Wednesday evening, and had been charged with intimidating a school staff member and behaving in an offensive manner.

The man, who was a juvenile at the time of the incident, was granted conditional bail to appear in a children's court on October 20.

A 14-year-old boy was arrested by police on Tuesday and will appear in a children's court on October 13 on the same charges.