Parents are writing vitriolic reviews of schools on the equivalent of a rate-a-restaurant website.
Headteachers have expressed concern about the potential for Schoolsnet.com to be used abusively - though analysis shows most of the 3,000 reviews are positive.
One parent, describing a school in a small Somerset town, said it was characterised by "packs of bullies roaming the playground attacking their victims at random".
Another, reviewing a north London comprehensive, wrote: "The teachers turn a blind eye, as the pupils who bring in the weapons are bigger than them."
Blakewater College in Blackburn has been attacked by one parent for being a "dumping ground for estate kids". Another review said that "most staff just seem to turn up every week for their pay cheque".
Lee Harris, Blakewater's head, was resigned to the attacks, saying schools had to put up with such comments, just as they would with critical letters to local newspapers.
"It's only people who have something to complain about who will write in," he said. "It's not a good investment of time to validate the attacks by responding to them."
While some of the criticisms are acerbic, the ratings are likely to give a more positive view of the school than other independent evaluations. Indeed, 61 per cent of parents rate their child's school as very good, and only 2.4 per cent rate it as very poor.
By comparison, Ofsted gave only 14 per cent of schools its top ranking of outstanding last year, and it rated 6 per cent as inadequate. Schools that are praised include Ridgeway School in Maidenhead, Berkshire, which has "an incredible caring ethos". Another parent compliments Newland House prep school in Twickenham, south-west London, writing: "I have seen all the children in my child's year grow in confidence in the past five years."
Greg Hadfield, Schoolsnet's creator, said it had rules to prevent parents from posting inappropriate reviews of schools.
Mr Hadfield, a former education journalist for The Sunday Times, previously founded Soccernet, a successful football news site, which he sold to Disney for pound;25 million.
He was inspired to set up Schools-net when he moved to Brighton and realised he was entirely reliant on word of mouth to decide which school his children should attend.
"It is important for any school to know what the gossip is at the school gate, whether good or bad," Mr Hadfield said. "If they don't know about it, they can't rebut it."
Mick Brookes, general secretary of the National Association of Head Teachers, said heads would be concerned about any such sites being used by parents with personal vendettas.
But, he suggested, parents could turn it around by using it as a positive forum for good news about individual children's progress.
WHAT THEY SAID
'The younger children are being neglected behind the scenes'
About a South Tyneside secondary school
'My son has been at MHS for 3 years now. It was certainly the best decision I ever made'
About a Nottingham prep school
'This is a brand new school, yet it has leaking ceilings, security doors that don't lock, secondhand books'
About a West Midlands primary school
'There's a warmth to Denmead School that you feel as soon as you walk up to the door'
About a Middlesex prep school
'Teachers gave me the feeling they couldn't care less'
About an Edinburgh primary school
All quotes taken from the Schoolsnet website: www.schoolsnet.com.