More than 200 schools, including 30 in the North West, are using CCTV cameras in toilets or changing rooms, figures have revealed.
A total of 825 cameras were located in the toilets or changing rooms of 207 schools across England, Scotland and Wales, showed figures provided by more than 2,000 schools.
Almost one in 10 of the schools which use CCTV said cameras were positioned in such places, while 54 have more than one camera for every 15 students.
Nick Pickles, director of Big Brother Watch, said: "This research raises serious questions about the privacy of schoolchildren across Britain, with some schools having one camera for every five pupils and hundreds of schools using cameras in toilets and changing rooms.
"The full extent of school surveillance is far higher than we had expected and will come as a shock to many parents. Schools need to come clean about why they are using these cameras and what is happening to the footage. Local authorities also need to be doing far more to reign in excessive surveillance in their areas and ensuring resources are not being diverted from more effective alternatives."
Big Brother Watch, which published the figures following a Freedom of Information Act request to more than 2,000 schools, also warned that the Home Office's proposals for a new regulatory structure was "not fit for purpose".
The new post of Surveillance Camera Commissioner "will have absolutely no powers to do anything", Mr Pickles added.
"Parents will be right to say that such a woefully weak system is not good enough."
Responses from 2,107 secondary schools and academies showed they used 47,806 cameras, including 26,887 inside school buildings. With 1.8 million pupils being taught in these schools, there was an average of one camera for every 38 children.
In all, 90% of schools had CCTV cameras, with an average of 24 cameras in each of the 1,537 secondary schools that responded and 30 cameras in each of the 570 academies. The estimated number of CCTV cameras in secondary schools and academies across England, Wales and Scotland was now 106,710, the campaigners said.