Mandurah students are being urged to take care when going to and from school following an incident on Thursday.
Authorities report a female year nine student from a local high school was on Transperth bus 594 to Halls Head/Erskine/Dawesville when she found no seats available.
An older man sitting on a single seat told the girl to sit with him.
The man continually harassed the girl and made inappropriate conversation.
The student left the bus at an earlier stop and reported the incident to her mother, her school and the police.
Students are urged to report any suspicious behaviour to parents and police.
Call 131 444 to make a police report.