Urgent guidelines on schools' internet use are needed as parents threaten to withdraw children from computer classes, a government-sponsored survey has found.
A study of ultra-fast broadband connections discovered confusion over how much internet access pupils should be allowed.
The report, by researchers at Nottingham Trent university, said: "There are those who are so concerned that they are unwilling to leave decisions regarding the internet to the school."
But researchers found fears of lax controls were not the only problem.
In one of the 37 schools surveyed, an excessively tight filter had prevented 14-year-olds looking at video clips about windfarms, planned as part of a geography lesson.
At Sir Frank Markham community school in Milton Keynes, which took part in the study, a filter system prevented pupils getting on to the Amnesty International website.
But Tony Hutt, the school's head of ICT, said: "I would rather have this than having to run round after children to check they are not on inappropriate sites.
"We have tried letting pupils use supervised chatrooms, but they want to go on the real thing. One pupil's parents have said they do not want their child to go on the internet at all."
The report, sponsored by the British Educational Communications and Technology Agency, said filters were vital in eliminating viruses and protecting pupils.
But it concluded: "National guidelines are necessary because of the very wide range of practice across the country, the impact of filtering on the efficient use of broadband and, the concerns of parents and teachers about unfettered access to the internet."
Fay MacDonald, communications co-ordinator at Internet Watch Foundation, a hotline the public can call to report racist or pornographic content on the web, said: "We would support anything which promotes a deeper understanding of the internet, but recommend children are monitored to the degree that an adult is there and available."