Past pupils at the £14,000 a year school include Chelsea footballer Frank Lampard, actor Keith Allen and TV presenter Noel Edmonds.
The school's head teacher, Ian Davies, wrote to parents to tell them about the expulsion, saying the pupil had been excluded for "using a mobile phone to take an inappropriate picture of a member of staff."
Mr Davies said the incident had "greatly upset many of our teachers."
The boy was initially suspended when he was seen photographing a female staff member, but after police found two more images on his phone, he was expelled.
The police are now said to be liaising with the school over complaints from teachers about the images.
According to the Brentwood Gazette, the boy had been initially suspended for taking a picture of a staff member two weeks ago.
But, following his readmittance to the school, further images of two other members of staff were found in circulation dating back to September 2011.
One of the three victims called in police to investigate the matter on January 31. The boy was expelled last week.
In a letter to parents, Mr Davies wrote: "Having carefully considered the evidence, and having secured further information this week, the pupil concerned will now leave the school.
"As a result of this incident, I intend to review our mobile phones policy, which is on display in every classroom around the school.
"Up until now, we have tried to balance the educational desire to ensure that students use the technology sensibly (although this sometimes seems to elude even adults) and the need to protect both students and teachers from the unwelcome effects of the misuse of this technology.
"Clearly, the incident has greatly upset many of our teachers, and I would welcome your thoughts on our current policy."
The new policy warns that children will be allowed to use mobile phones only during break and lunchtime, and then only for "school-related matters and essential communication from home" and are banned from using them in the dining hall and library and from taking photographs, creating moving images or recording schools.