More than 200 schools are using CCTV cameras in toilets or changing rooms, figures showed on Wednesday.
A total of 825 cameras were located in the toilets or changing rooms of 207 schools across England, Scotland and Wales, figures provided by more than 2,000 schools showed.
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."
Hardial Hayer, headteacher at The Radclyffe School, which has 20 cameras, said the CCTV was in place for safeguarding children.
"The cameras do not have any impact on anybody's privacy," he told The Huffington Post UK. "They just see students entering the toilets. We are open about having the cameras - we tell all the parents and the students have said they feel safer.
"Very often children congregate in the toilets and make other pupils feel intimidated. With the cameras we always know how many are in there.
Big Brother Watch, which published the figures following a Freedom of Information Act request, 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", 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.
The 10 schools with the most cameras were:
1) The Radclyffe School - North West - Oldham - 20
2) St Mary's Church of England High School (VA) - East of England - Hertfordshire - 18
3) The Friary School - West Midlands - Staffordshire - 14
4) Wildern School - South East - Hampshire - 12
5) Ecclesfield School - Yorkshire and Humberside - Sheffield - 12
6) Portland School - East Midlands - Nottinghamshire - 12
7) King Ecgbert School - Yorkshire and Humberside - Sheffield - 12
8) Montgomery High School - North West - Blackpool - 12
9) St Wilfrid's Catholic High School and Sixth Form College - Yorkshire and Humberside - Wakefield - 12
10) Thomas Deacon Academy - East Midlands - Peterborough - 12
Headteacher Hayer added the reason the school is at the top of the list is due to having 20 small toilet blocks, rather than a few large ones.
"The cameras mean the school is a safe environment. It has been a huge success."
A spokesperson for the National Association of Head Teachers (NAHT), said schools that use CCTV must ensure the cameras "are placed sensitively".
"It is important that parents and pupils are fully aware that cameras are present and that issues of who monitors, and what happens to, the resulting footage are clear in the home-school agreement," the spokesperson said.
"It is always a balance between preserving an individual’s right to privacy with their right not to be maliciously targeted in inaccessible areas of the school.
"Most schools will be aware of these conflicts and aim to manage each situation on its merits."
A Department for Education spokesperson said: "Schools using CCTV are required by law to adhere to the Data Protection Act.
"We have already acted to make it unlawful for schools to use biometric data like fingerprints without parents' permission. CCTV can be beneficial in some cases but this is a decision that head teachers should take."
The school with the least students per camera was Christ The King Catholic and Church of England (VA) Centre for Learning, Knowsley, in the North West of England, which had five students per camera.
The school could not be reached for comment.Suggest a correction