Girl sexually assaulted at Vic school

A young girl has been sexually assaulted after a man exposed himself to her and a friend at a Melbourne primary school.

The two nine-year-old girls from Laburnum Primary School were on their way to the toilet at 11.30am on Monday when the man exposed himself, police say.

It is alleged the man then sexually assaulted one of the girls before they ran away.

The students told a staff member who immediately contacted police.

"We are taking this very seriously and have introduced a number of interim safety measures to further protect our students," principal Steve Richardson said in a statement on Tuesday.

The school is working with the Box Hill Sexual Offences and Child Abuse Investigation Team who are investigating the incident.

Police say the attacker was of Asian appearance, aged in his 20s, with an average build and black hair which was longer on the top and shaved at the sides.

He was wearing a dark-coloured long sleeve top, dark trousers and white Adidas runners.