Police have tasered a man who threatened a 13-year-old schoolgirl with an iron bar on Sydney's northern beaches this afternoon.

The year seven student was threatened with the bar by a man who stormed into the quadrangle of Manly Selective High School around 2:30 this afternoon.

Police arrived within minutes but the man became more aggressive hitting one officer in the arm with the iron bar, then biting him.

Other officers doused the attacker with capsicum spray and tasered him.

Tonight he's under guard in Manly hospital.

The incident was terrifying for parents who were sent text messages saying that the school was in lock-down.