A man allegedly manhandled a girl in Plumpton on Tuesday.
The 15-year-old girl was standing outside her school in Hyatts Road about 11.15am.
Police were told the man approached her from behind, placed his hand across her mouth and touched her body.
She broke away from him and ran into the school, where she reported what happened.
Police are investigating and said they want to speak to a man described as being of Mediterranean or Middle Eastern appearance, in his mid-30s, 170-175cm tall, of solid build, with a beard and black hair with grey regrowth.
He was also wearing a black hooded-jacket and grey track pants with white stripes down the sides.
Anyone with any information is urged to contact Mount Druitt police on 9675 0699 or Crime Stoppers on 1800 333 000.
Police also said parents and carers should urge their children to remain safe when out and, if unaccompanied, keep away from people they do not know.