Police are investigating claims that a teenage boy sexually assaulted a girl at a house party in Sydney's eastern suburbs in an attack that was filmed on a mobile phone and shared on social media.

A 15-year-old boy has been charged by police after he allegedly filmed the assault in Bellevue Hill on March 4 and posted it online.

Detectives will question the boy's friend, also aged 15, over claims he sexually assaulted the 15-year-old girl at the party. Neither boy can be named for legal reasons.

The boy accused of assaulting the girl was a student at the elite Cranbrook School in Bellevue Hill and teachers at the school reportedly raised the alarm when they became aware of the allegations.