Around 1,000 secondary schools are using computer software to monitor pupils, it has been suggested.
Campaigners have raised concerns that while many schools are putting these measures in place to help protect children, many parents and students are unaware that the technology has been installed. They also warned that there is a risk that monitoring and filtering of online content can go too far.
Figures obtained under Freedom of Information requests by civil liberties group Big Brother Watch show that of 1,420 secondary schools in England and Wales which provided data, 1,000 said that they use so-called classroom management software packages.
This technology can be used by schools in a number of ways, such as to monitor pupils' internet activity, access their web history, block websites and check what is being typed into a computer.
Around £2.5 million was been spent by these schools on this software, which was installed on a total of 821,386 computers, laptops, tablets and mobile phones.
Out of these 1,000 schools, 149 - around one in six - provided acceptable use policies, and of these, 26 gave information about the type of software and how it was used. A total of 123 did not give any further details beyond stating that students may be monitored when using computers.
In its report, Big Brother Watch said that schools often consider buying the software to keep pupils safe online, or as part of their duty to help protect children from radicalisation.
But it added: "We are concerned that the use of technology which allows real time monitoring is placing teachers unwittingly in the position of being Big Brother.
"Forcing staff to oversee their pupils' every digital move represents a fundamental shift from the traditional method of overseeing pupils by engaging with them from the front of the class."
It went on to say: "Schools currently offer little explanation about the use of the software in their acceptable use policies. Pupils and parents, who have to sign such policies to say they agree to the use of the software, are therefore left completely in the dark."
Renate Samson, chief executive of Big Brother Watch said: "Finding the balance between keeping pupils safe online without impinging on their right to privacy is a challenge for every school. But encouraging schools to track and monitor pupils creates a worrying precedent, particularly if pupils and parents are being left in the dark."
Malcolm Trobe, interim general secretary of the Association of School and College Leaders, said: "Computer monitoring software is used in schools to safeguard the welfare of children and young people by ensuring that they are not exposed to damaging online material.
"There is no secrecy about the use of this sort of software in schools. Pupils are very much aware of rules about computer use and most schools have policies which are available to parents."
A Department for Education spokeswoman said: "Schools have a responsibility to keep pupils safe, including online, and schools should use appropriate filters and monitoring systems to protect children from harmful material.
"How individual schools decide to do this is rightly a matter for the school, engaging with pupils and parents as appropriate."