Pupils in more than 200 schools can be seen by CCTV cameras when they enter toilets or changing rooms, new research has revealed.
The figures, obtained by privacy campaign group Big Brother Watch, also reveal that more than 50 schools have at least one camera for every 15 students.
The survey, based on figures provided by some 2,000 schools, show a total of 825 cameras were located in the toilets or changing rooms of 207 schools across England, Scotland and Wales.
The figures showed that 54 schools have more than one camera for every 15 students and some have one camera per five pupils.
Nick Pickles, director of Big Brother Watch, said: "This research raises serious questions about the privacy of schoolchildren across Britain."
He added: "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 rein in excessive surveillance in their areas and ensuring resources are not being diverted from more effective alternatives."
Freedom of Information Act 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.
The total estimated number of CCTV cameras in secondary schools and academies across England, Wales and Scotland is now 106,710, the campaigners said.
The Radclyffe School in Oldham, Greater Manchester, topped the list of schools, with 20 cameras in toilets or changing rooms.
Head teacher, Hardial Hayer, said its cameras are above doors entering toilets, only overlook washbasins and are not near cubicles.
He said the high number reflects the fact that the building has smaller toilet blocks than many schools and is a bigger school.
Siobhan Freegard, co-founder of parenting website Netmums, said: "I think everybody realises that this is not in the cubicles, it is in the open areas, and actually, that's where a lot of bullying and difficult behaviour takes place.
"It is an area where teachers are not likely to be. Parents are probably quite pleased about it."
A Department for Education spokesman said: "CCTV can be beneficial in some cases but this is a decision that head teachers should take.
"Schools using CCTV are required by law to adhere to the Data Protection Act."
The full report can be downloaded at http://www.bigbrotherwatch.org.uk/