Police hunt for intruder behind sexual assault at Melbourne primary school

Twenty-four schools across Melbourne's south-eastern suburbs are on high alert after a horrific sex attack on a nine-year-old girl at a greater Melbourne primary school yesterday morning.   

The Laburnum Primary student was allegedly sexually assaulted by an intruder around 11:30am yesterday.

The attacker is believed to have exposed himself to the girl and another nine-year-old girl as they walked towards a toilet block.

He then allegedly grabbed one of the victims and sexually assaulted her, but the girl was able to break free and flee with her companion.

It's understood that the assault occurred in a section of the school adjacent to bushland.

Police have spent today combing large sections of the area outside of the school grounds where the man is said to have fled. 

The suspect is believed to be of Asian appearance, aged in his 20s, and was dressed in a long-sleeved, dark-coloured top, dark tracksuit pants and white Adidas sneakers.

He is believed to have entered and exited through an open gate.

Principal Steve Richardson said the school has enacted "interim safety measures", with parents informed of the incident in writing yesterday.

"The safety of students is always our top priority," Mr Richardson said in a statement.