Schools are being offered guidance that suggests that having consensual sex at the age of 13 is normal behaviour, it has been claimed.
Sarah Carter, of the Family Education Trust, told a cross-party group of MPs she believed that sometimes what is taught in school sex and relationship lessons is not lawful.
Giving evidence to the Commons education select committee, Ms Carter highlighted recently-published supplementary guidance on sex and relationship education (SRE), which included a link to a "Traffic Light Tool" from sexual health and advice service Brook.
She said that Brook's tool "states that young people who are consensually sexually active from the age of 13, this is normal behaviour and development, whereas actually the law states that young person should wait until they are 16 at least, never mind if they are ready or not", Ms Carter said.
"That's awfully unlawful behaviour, and so quite often what's taught in SRE isn't always lawful."
The supplementary guidance on sex and relationships education (SRE), which contained links to a number of resources that could be used by schools, including the Traffic Light Tool, was developed and published by the PSHE Association with Brook and the Sex Education Forum.
Also giving evidence, Joe Hayman, chief executive of the PSHE Association, said: "I think the only challenge with dealing with these subjects, and this is why we need really well-trained teachers, is that we've got to deal with children's realities."
Committee chairman Graham Stuart told Mr Hayman that critics might argue that to send out messages that 13-year-olds having sex together is part of growing up and "not to send out a message that it's wrong, that it's harmful, it's dangerous, is in fact to almost to collude with something which we know is damaging to young people".
Mr Hayman said: "What I was saying was that it's really, really important that a dictatorial-from-the-front lesson on what one should and shouldn't do is less likely to have an impact and I think we've got to start from where children are, their reality.
"There's no one in our community who feels we should be trying to sexualise children, or any of those kinds of things.
"What we want is children to develop healthy and safe relationships and it's really important that teachers are provided with the necessary training in order to do that."
He added that all the resources that the PSHE Association produces are clear about teaching about the law.
Questioned further about Brook's Traffic Light Tool, Mr Hayman insisted it was difficult for him to be accountable for every piece of information linked to in the PSHE Association's supplementary guidance, which has many links within it. He agreed to write to the committee about the issue.
Brook's Traffic Light Tool on sexual behaviours, which is available online, is meant to help professionals assess whether children and young people's sexual behaviours are healthy or unhealthy.
It sets out green, amber and red "behaviours" for different age groups.
Under the 13-17 age group, it lists a number of green behaviours, which are described as behaviours that reflect "safe and healthy sexual development" displayed between children and young people of similar age and developmental ability and "reflective of natural curiosity, experimentation, consensual activities and positive choices".
The list includes "having sexual or non-sexual relationships", "sexual activity including hugging, kissing, holding hands", "consenting oral and/or penetrative sex with others of the same or opposite gender who are of similar age and developmental ability" and "choosing not to be sexually active".