A scheme to help teachers distinguish between normal sexual experimentation and abuse by other pupils is being piloted by a children's charity.
A third of all sex crimes are committed by children, so teachers need to be vigilant, says the National Society for the Prevention of Cruelty to Children.
However, some acts have to be viewed in context and the Aim (assessment, intervention and moving on) pilot scheme in Manchester looks at how to identify and manage sexually problematic behaviour.
Peeing contests between boys and the "I'll show you mine if you show me yours" games have been played out in school playgrounds since time immemorial. And the charity wants to ensure schools do not treat young people as offenders if they indulge in normal sexual experimentation.
Julie Henniker, the scheme's manager, said that some behaviour which would be worrying among under-10s could be normal for adolescents. "It can also depend on how the pupils react afterwards," she said. "If two children are caught naked together and they are both laughing and joking, it is different from a case where one seems unhappy and frightened."
Ms Henniker and child protection expert Carol Carson have developed three categories for pupils' sexual behaviour:
* Healthy: behaviour which is considered age appropriate, leaves other children involved joking or slightly embarrassed, and is usually between children who normally play together.
* Problematic: behaviour which can be recurring, age inappropriate and leaves children feeling uncomfortable.
* Abusive: behaviour which involves adult sexual activity (such as oral sex between primary pupils), or anything that frightens a child.
Solutions for tackling problematic and abusive behaviour range from brief talks with parents to calling in social services.
In Manchester, Aim has held "strategy meetings" for some pupils with abusive tendencies in which representatives from a range of agencies have met the young person and their families to come up with a plan to stop them re-offending.
Ms Henniker said it was hard to tell whether sexually harmful behaviour by pupils was on the rise. Reports of such cases are increasing, she said, but this might be because teachers were better at identifying them.
Katie Rowley, a religious education teacher at St Thomas A Beckett school in Wakefield, west Yorkshire, is keen to take part in the scheme. She said she was convinced pupils were becoming more overtly sexual. On more than three occasions, teenage boys had masturbated in front of staff at her previous school.
The NSPCC hopes to expand the Aim project to schools in a wider range of authorities from Wednesday when it will be holding Children's Day, an event to promote child protection issues.
Carol Close, a Wigan headteacher who took part in Aim, said schools would find the course reassuring. "After some child protection courses you start worrying that three children in every class are being abused," she said.
"This reminds you that a lot of sexual behaviour can be normal and healthy."
SORT THE NORMAL FROM THE NASTY
Is this behaviour healthy, problematic or abusive?
1 Two seven-year-old boys are showing their penises to each other in the school toilets and urinating up the walls. When found they are embarrassed.
They do not repeat the behaviour.
2 Two nine-year-old girls are found in the cloakrooms, one has her pants down. This girl does not appear to be happy and it is known that she is scared of the other girl. The other girl denies doing anything.
3 A nine-year-old boy who is known to bully other children is found in a secluded part of the playground with a seven-year-old boy. The younger boy later disclosed that the nine-year-old wanted him to "kiss his willy".
4 A group of eight-year-old boys chase an eight-year-old girl in the playground and try to pull her skirt up and her pants down. She is very upset by this and tells a lunchtime supervisor.
Co-ordinators of the Aim project use these scenarios to provoke discussion, and say there are no definite answers