Blue School pupils were behind the placing of CCTV in the toilets, much to the initial shock of head teacher Steve Jackson.
Figures released this week show that the Blue is one of 23 schools in the west of England that have installed cameras in toilets or changing rooms.
The information came to light after a series of Freedom of Information Act requests from privacy pressure group Big Brother Watch.
Nick Pickles, director of Big Brother Watch, said: "Schools need to come clean about why they are using cameras and what is happening to the footage.
"Local authorities also need to be doing far more to rein in excessive surveillance in their areas and ensuring resources are not being diverted from more effective alternatives."
However, Mr Jackson said the Blue had CCTV installed in the toilets seven years ago and that it was students who wanted the cameras put in place, not the school.
"I was frankly horrified when the students from our Toilet Team came to us with this idea," he said."
The Toilet Team had been undertaking redecoration and refurbishment of the toilets and wanted to make sure their hard work was not undone.
They worked closely with the school's Leadership Team and ICT department to decide on the best way to go about the installation.
Mr Jackson said: "I anticipated negative reactions from the media and parents alike. But it was the students who arranged everything and even broke the news to their parents. In seven years we haven't had one objection."
Mr Jackson said the cameras had been put in place to stop the toilets being used inappropriately and that there is no real-time viewing. The footage is only looked at if there is an allegation of an incident by a student and then only by a very senior member of staff.
He added: "It's passive supervision. The cameras are modestly aligned, they only film the sink area to show who's going in and out of the cubicles, and the footage is only seen by a very small amount of people."