Police are investigating five year 9 boys from a prestigious Catholic boys' college on Sydney's northern beaches who shared sexual images of two female teachers on Facebook.

The boys, from St Augustine's College at Brookvale, used iPhones to photograph and video up the skirts of the two teachers and then shared the images on a specially created thread on Facebook.

The boys were suspended from the school, while another five boys who followed the Facebook thread have been "disciplined". It is understood the offenders have had "re-enrolment" interviews and have returned to the school.

Fairfax Media has been told that the boys had been capturing the images for several months.

In a statement to staff from the college's head of school, Matthew Hutchison, the boys' behaviour was described as "calculated". The statement said the teachers had been "betrayed" by the boys, who had committed a criminal offence.