Children are being bullied into providing "sexual favours" in exchange for protection as gang culture enters schools, campaigners have warned.
Beatbullying is running a project on sexual bullying
Charity Beatbullying said it had seen growing evidence of sexual bullying between boys and girls at school.
Chief executive Emma-Jane Cross said some girls in very rough schools were pushed into sex acts for protection.
But other anti-bullying groups and teachers' unions challenged the charity to provide evidence of this.
The warning comes as the House of Commons Education Select Committee published a report highlighting weaknesses in the way schools tackled bullying.
Speaking after the report was published Ms Cross called for an urgent government investigation into the "sexual exploitation of young people by young people".
But her charity's claims were questioned by the National Union of Teachers, which said it had no evidence that this sort of behaviour was happening in schools.
General Secretary Steve Sinnott said: "If they [Beatbullying] know this is happening, they should tell the schools concerned.
"It has to be stamped out. But it is very difficult for teachers to prevent appalling behaviour beyond the school gates.
"I hope Beatbullying isn't using exaggerated language of 'sexual favours' when they mean sexual harassment - name calling, pinching, touching inappropriately."
And the director of Kidscape, Dr Michele Elliott, said her children's charity was in contact with two million children and parents but had not "received such allegations".
"If it is happening on such a wide scale, it needs to be documented and addressed.
"If it isn't, then it is irresponsible headline grabbing and reflects badly on any organisation that would make such a claim."
"If it is true then let's find this out," she told the BBC News website.
Comic Relief funding
Director of Bullying Online Liz Carnell said the charity had not had a single complaint of sexual favours being demanded in exchange for protection against bullies in the 48,000 emails it received in eight years.
"We are sure that if this was happening to the extent alleged we would have heard about it, if not from the girls involved, from other children.
"This is an incredibly serious allegation involving a child protection issue and Beatbullying needs to provide the evidence of which schools are involved so that the head teachers, governors, the LEA and police can take action."
A spokesman for Beatbullying said: "I can assure you we do have the evidence - it may be anecdotal and small but it is shocking and real."
He added that the charity was now being funded by Comic Relief to run a sexual exploitation and bullying programme looking at this worrying development.
The programme had however only just started and this was why the evidence was anecdotal, he said.
But once it had progressed the findings would be summarised in an academically robust report, he added.